In all languages, the terminology of law is cryptic and difficult to navigate, and Irish is no exception. Many of these words will probably sound clumsy and artificial. However, it is quite important to learn to use legal terms in Irish. Here is a list. (Of course, in the long run we need court dramas in the Irish language, so that the terms can be learned in context.)
“an accomplice” – for this we have two terms, comhchoirí (masculine, plural comhchoirithe) and neartaitheoir (masculine, genitive neartaitheora, plural neartaitheoirí). As far as I know, there is no difference in meaning between the two terms. For an “accessory to crime”, there is another word, cúlpháirtí.
“to acquit” – éigiontaigh!/éigiontú
“an affidavit” is, as you should know, a sworn statement, and the word for that is mionnscríbhinn. This is a combination of mionn (oath) and scríbhinn (writing), and it is obviously femininen (an mhionnscríbhinn, na mionnscríbhinne, na mionnscríbhinní, na mionnscríbhinní). Swearing by the Bible or other holy book is called (idiomatically, not juridically) an leabhar a thabhairt, i.e. to give the book. If you do not want to swear by the book, you give an affirmation, in Irish dearbhascadh. (This is basically the verbal noun of the verb dearbhasc!/dearbhascadh ‘to affirm’.)
“to appear in court” is láithriú i gcúirt. “He appeared in court”: Láithrigh sé i gcúirt.
“a barrister” is abhcóide, as Eimear pointed out in the comments. Thanks! Go raibh míle maith agat!
“to commit, to perpetrate”: no special verb is needed. Use déan!/déanamh. “He committed a heinous crime”: rinne sé coir ghránna. “It never occurred to me that he should have perpetrated such a crime”: níor rith liom riamh go ndéanfadh sé a leithéid de choir.
“common law” is an dlí coiteann, and note that in Irish it takes the definite article. In English, generic abstract nouns typically omit the article, but in Irish, they take the article. That is why we have an Ghaeilge, while English uses just Irish. (And that is also why they often speak of “the Gaelic” in Scotland – that is an Irish or Sc. Gaelic article usage carried over to English.)
“a complainant” is gearánach, “a complaint” is gearán, and “to withdraw a complaint” is gearán a aistarraingt. “He withdrew his complaint” = d’aistarraing sé a ghearán.
“a court (of law)” is cúirt, of course – an chúirt, na cúirte, na cúirteanna, na gcúirteanna. A district court is cúirt dúiche, a circuit court is cúirt chuarda, a high court is ardchúirt. The weird word cuarda is a genitive form of cuaird (feminine noun), which means “circuit” in the juridical sense. There are eight juridical circuits (i.e. circuit court jurisdictions) in Ireland: Cuaird Átha Cliath, Cuaird Chorcaí, Cuaird Lár na Tíre, Cuaird an Tuaiscirt, Cuaird an Oirthir, Cuaird an Iardheiscirt, Cuaird an Oirdheiscirt, Cuaird an Iarthair.
“custody” (of a child) is as legal term coimeád. “He has the custody of the child”: tá coimeád an linbh aige.
“a deed poll” (plural: deeds poll) is the legal expression of an intention. Typically, if you want to change your name from, say, John Smith (or Johann Schmidt, or Jan Kowalski, or Ivan Kuznetsov, or Juha Seppänen) to Seán Mac Gabhann, you do it by deed poll. The Irish term is gníomhas aonpháirtí, i.e. one-party or one-sided deed. Gníomhas means ‘deed’ as a legal term. Deed poll as the way to change your name is typical of common law countries, where your legal name is based on common repute. Gníomhas is a masculine noun (genitive and plural: gníomhais), the aonpháirtí part is technically the genitive form of a noun.
“ejectment” of a tenant is eisiachtain (feminine, genitive eisiachtana). “The court ejected the tenant.” D’eisiacht an chúirt an tionónta. Note that this verb only refers to court proceedings – the concrete task of ejecting or evicting a tenant is tionónta a dhíbirt, a chur amach. So, the bailiffs evicted the family: chuir na baillí an teaghlach amach, dhíbir na baillí an teaghlach. Historically speaking, there is also the verb díshealbhaigh!/díshealbhú “to evict, to dispossess”: Dhíshealbhaigh na Gaill an teaghlach Gaelach. “The wicked Englishmen evicted the Irish Gaelic family.”
“execution”: As a legal term, it is forghníomhú: “the sentence was executed”: forghníomhaíodh an bhreith. However, note that forghníomhaigh!/forghníomhú “to execute” is pure legalese and will not be understood by those who only speak everyday Irish. They will prefer such expressions as cur i gcrích or comhlíonadh. When this verb is used as a eufemism for putting somebody to death, it is best translated into Irish with básaigh!/bású, or with the exact Irish equivalent of “put to death”, i.e. cuir!/cur chun báis. Chuir na saighdiúirí dearga an sagart chun báis “the redcoats executed the priest”.
“a felony” – feileonacht. It is of course feminine: an fheileonacht, na feileonachta, na feileonachtaí, na bhfeileonachtaí. It is not an official legal term in Ireland anymore – instead, tromchion and cion inghabhála are preferred.
“Grand Jury” is not an Irish institution of law anymore, but if you need to speak of American circumstances or of Irish history, the term is an tArd-Ghiúiré. As you see, it is masculine. For “Federal Grand Jury”, I would use Ard-Ghiúiré na Cónaidhme, although there is nothing wrong with An tArd-Ghiúiré Feidearálach.
“an indictment” is díotáil (feminine: an díotáil, na díotála), and the verb “to indict” is obviously díotáil!/díotáil. “To issue an indictment” is díotáil a eisiúint, “Grand Jury issued an indictment” is d’eisigh an tArd-Ghiúiré díotáil.
“an injunction” is a court order (ordú cúirte) telling you to stop or start doing something specific. The Irish term for an injunction is urghaire, and it is feminine (an urghaire, na hurghaire, na hurghairí, na n-urghairí).
“a judge” is breitheamh. This is a somewhat irregular masculine: an breitheamh, an bhreithimh; na breithiúna, na mbreithiún. Note that a justice of the peace is giúistís (an giúistís, an ghiúistís; na giúistísí, na ngiúistísí). A dialectal word for the latter (used by the Ulster author Seán Bán Mac Meanman) was giúsach péas.
“jurisdiction” is dlínse, and that is a feminine word.
“a jury” is giúiré (masculine), or in a more generic sense coiste cúirte. A juror, a member of the jury, is called giúróir (masculine: an giúróir, an ghiúróra, na giúróirí, na ngiúróirí).
“a lawyer” is dlíodóir or in books written by Ulster authors dlítheoir (masculine, genitive dlíodóra, dlítheora, plural dlíodóirí), a solicitor is aturnae (masculine, plural form aturnaetha), which is of course the same word as attorney.
“a legal remedy” is réiteach dlí.
“a litigant”, i.e. a party to a case, is dlíthí, masculine, with the plural form dlíthithe. If you learned your Irish in Connemara, that word will put you in umar an éadóchais, I am afraid.
“a manslaughter” is called dúnorgain, and that is a feminine noun. The genitive form is dúnorgana.
“a misdemeanour” – oilghníomh. It is a masculine noun: an t-oilghníomh, an oilghnímh, na hoilghníomhartha, na n-oilghníomhartha. Note that it is not an official legal term in Ireland anymore (the utterly clumsy neamh-thromchion and cion neamh-inghabhála are preferred in officialese).
“a murder” is dúnmharú, as everybody should know already. However, the verb dúnmharaigh!/dúnmharú is kind of heavy and clumsy; maraigh!/marú is usually preferable. A murderer is obviously dúnmharfóir; I’d call a spree killer spraoimharfóir, a serial killer sraithmharfóir.
“an offense” is cion, a masculine noun (men commit most crimes!), genitive ciona, plural cionta. There is of course the feminine word coir too, which means simply “crime”.
“parole”: “on parole” is either ar parúl or ar d’onóir (ar m’onóir, ar d’onóir, ar a onóir, ar a honóir), both are mentioned in Ó Dónaill’s dictionary. Ar parúl is again an example of ar not eclipsing the first consonant of a following naked noun when it’s about an abstract situation, not a concrete position. (Similarly: ar fionraí, ar promhadh, ar tarraingt “in traction”…)
“a plaintiff” is gearánaí (masculine, plural is gearánaithe). It is regrettably very easy to mistake for gearánach, complainant.
“a plea bargain”: the online terminology database gives margáil pléadála, which to me suggests the act of plea-bargaining. I am quite tempted to call the result margadh pléadála.
“to plead guilty/innocent”: pléadáil ciontach/éigiontach. “He pled guilty.” Phléadáil sé ciontach.
“police” is in the Republic of Ireland called An Garda Síochána or the Guardian of Peace; police officers are accordingly spoken of as gardaí; even in Irish English the term “guards” is reportedly used. Note that the Irish word for peace (síocháin, feminine, gen. síochána; in Ulster sometimes the masculine síochamh, gen. síochaimh) also means public order. The English word “peace” has actually been borrowed into Irish as péas – this is indeclinable, except by initial mutation: an péas, an phéas; na péas, na bpéas – and means “policeman”. Moreover, there is the word póilín, which is probably the most neutral word for a policeman. From this we have the verb póilínigh!/póilíniú “to police” and the noun póilíneacht. A police force is in Irish a policing force, fórsa póilíneachta. A constabulary is constáblacht – as in Constáblacht Ríoga na hÉireann “the Royal Irish Constabulary”, the members of which were called in Irish na saighdiúirí dubha “the black soldiers”, although strictly speaking their uniforms were dark green (dubhuaine), not black (dubh). (This was to distinguish them from the British Army, which was called na saighdiúirí dearga, the red soldiers, i.e. redcoats. Note that even the expression an tArm Dearg “the Red Army” in Irish suggests the British Army, at least in 19th century folklore.)
“probate”: the noun is probháid, and it is feminine. Usually it is used just as a genitive attribute, such as in ábhair phrobháide “probate matters”.
“probation”: this is promhadh, masculine, genitive promhaidh. “To be on probation”: bheith ar promhadh. Note that in phrases like this, which refer to abstract situation rather than concrete position, ar does not lenite the following word (although there is an exception to this exception, ar fheabhas).
“to prosecute”: ionchúisigh!/ionchúiseamh. The prosecutor is an t-ionchúisitheoir, and the Chief Public Prosecutor is an Príomh-Ionchúisitheoir Poiblí.
“a safety order” is one of the documents a court can issue to fight domestic violence, and in Irish it is called ordú sábháilteachta. The related concept of “barring order” is in Irish ordú urchoisc.
“a summons” is officially toghairm, which is a feminine noun, but this is a very literary, classical word, and gairm dlí is preferable. A summons is served, and the verb for “to serve” is in this, and only this, sense seirbheáil!/seirbheáil. An “originating summons” is toghairm thionscnaimh, a “plenary summons” is toghairm iomlánach, a “summary summons” is toghairm achomair, and a “special summons” is toghairm speisialta. A “subpoena” is even in Irish subpoena, as the word is basically unadulterated Latin.
“tort” is tort in Irish too: an tort, gen. an torta, plural na tortanna.
“a trial” is triail, and it is feminine, having the genitive form trialach and the plural trialacha. “To stand trial” is triail a sheasamh according to the standard dictionaries, but note the vintage Ulster expression an dlí a sheasamh.
“a tribunal” is binse (masculine: an binse, an bhinse, na binsí, na mbinsí – and yes, it is the same word as the Irish for “bench”).
“a verdict” is fíorasc, but it is again a purely legal term that won’t be understood by everyone. It is masculine: an fíorasc, an fhíoraisc, na fíoraisc, na bhfíorasc.
“to waive” – tarscaoil/tarscaoileadh. “He waived the claim” – tharscaoil sé an t-éileamh. “A waiver” is tarscaoileadh as a noun.
THIS VOCABULARY IS A WORK IN PROGRESS. PLEASE SUGGEST ADDITIONS IN THE COMMENTS.