The Irish are not really a warlike people today, but they have been, and many of those Irishmen who emigrated to An tOileán Úr ended up in warlike professions. Also, the military prowess of the Irish shown in the American Civil War – Cogadh Cathartha na Stát Aontaithe (“the Civil War of the United States”) or Cogadh Cathartha Mheiriceá (“the Civil War of America”) – was an important part of the Irish being accepted as fellow Americans, which in the times of no Irish need apply wasn’t quite obvious yet.
Anyway, war is cogadh, masculine, genitive cogaidh, plural cogaí (although I am sure such double plurals as cogaíocha are perfectly acceptable in spoken language). A soldier is saighdiúir, gen. saighdiúra, plural saighdiúirí. Traditionally, saighdiúir dubh or “black soldier” used to mean a member of the Royal Irish Constabulary (Constáblacht Ríoga na hÉireann), and a saighdiúir dearg was not a Communist guardsman, but a redcoat, a British soldier. (Even An tArm Dearg referred to the British army.)
The Anglo-Irish war can be called an Cogadh Angla-Éireannach, but also Cogadh na Saoirse, the war of freedom, liberty or liberation. The Irish civil war can be called Cogadh Cathartha na hÉireann, but more idiomatically, Cogadh na gCarad, the war of the friends.
The adjective “military” is míleata. (“Warlike” is cogúil, cathach or trodach.) The noun, “the military”, must be rendered with an tArm, “the army”. And if you join the army, you “go into it”: chuaigh an fear óg san arm (ins an arm), chuaigh an fear óg sna saighdiúirí (ins na saighdiúirí) “the young man joined the army”.
Ammunition can be muinisean with an English loanword (masculine, genitive muinisin), but there are also such expressions as armlón, lón lámhaigh, and lón cogaidh. The last one I would prefer myself, it has wide currency in literature. It might be a little inexact though; according to the Ó Dónaill dictionary, it can also be used in the more general sense of war supplies: ordanás agus lón cogaidh is defined as “ordnance and supplies” in Ó Dónaill under ordanás.
A gun is called gunna, masculine, with the plural form gunnaí. A rifle is raidhfil, masculine, with the plural raidhfilí. A shotgun is gunna gráin. Artillery guns are called gunnaí móra – simply “big guns”. Artillery is of course airtléire. The verb for shooting is usually scaoil!/scaoileadh, “to fire a shot” is thus urchar a scaoileadh. Loisc!/loscadh is also possible; in the sense of shooting someone, you can use caith!/caitheamh – for instance, caitheadh Pádraig Mac Piarais i ndiaidh Éirí Amach na Cásca “Patrick Pearse was shot after the Easter Rising”. Also for firing other kinds of projectiles, such as flares: a flare is lasrachán, to fire a flare is lasrachán a scaoileadh. Note though that lasrachán is not used in authentic Irish-language memoirs from the Anglo-Irish war: even in Irish-language contexts flares are called Very lights. A shot, a round of ammunition is urchar.
Conscription is coinscríobh, masculine, genitive coinscríofa. This is a modern neologism based on the English word. Back in Séamus Ó Grianna’s times, the common people translated it preasáil, which though is not strictly the same as conscription, but means impressment, press-ganging, shanghaiing, forced recruitment of soldiers or seamen. (In Seán Bán Mac Meanman’s collected works, you will find many folklore-based stories about the cidnappers – thus written! – who kidnap young Irishmen for the navy.) A conscript(ed soldier) is coinscríofach.
One of the early attempts to translate “conscription” into Irish was óglachas éigeantach. The word óglach, which even people with scant knowledge of Irish will recognize, means “volunteer” in the military sense. It is originally óglaoch (“young hero”); in poetry, the variant óglách is also used if needed for rhythm or rhyme. Óglaigh na hÉireann “the Volunteers of Ireland” or “the Irish Volunteers” has today a strong nationalistic overtone, and both the army of the Irish state and various IRA-related outfits call themselves so, which is the reason why the IRA is simply called an tIRA in Irish, too. The Irish defence forces in their entirety call themselves Óglaigh na hÉireann, but for clarity also Fórsaí Cosanta na hÉireann; they comprise the Army or Arm na hÉireann (or an tArm, if not followed by the na hÉireann attribute), the Naval Service or Seirbhís Chabhlaigh na hÉireann (or an tSeirbhís Chabhlaigh, when the na hÉireann part is omitted) and the Air Corps (Aerchór na hÉireann or an tAerchór).
Recruitment is earcú, the verb is earcaigh!/earcú: Tá an tArm ag earcú daoine óga díograiseacha chun an tír a chosaint “The Army is recruiting enthusiastic young people to defend the country”. A recruit is earcach, and can be called rúcach or a rookie (the Irish word is an established loanword). Another word is fruiligh!/fruiliú, but it is seldom used in a military context – I remember to have seen it once or twice used in that sense, though.
A noncommissioned officer is in Irish oifigeach neamhchoimisiúnaithe, too. An officer is oifigeach.
The army ranks are earcach (recruit), saighdiúir singil (private), ceannaire (“leader” – i.e. corporal), sáirsint (sergeant), ceathrúsháirsint complachta (company quartermaster sergeant), sáirsint complachta (company sergeant), ceathrúsháirsint cathláin (battalion quartermaster sergeant), ceathrúsháirsint reisiminte (regimental quartermaster sergeant), maorsháirsint cathláin (battalion sergeant-major), maorsháirsint reisiminte (regimental sergeant-major), dalta (cadet), dara leifteanant (second lieutenant), leifteanant (lieutenant), captaen (captain), ceannfort (commandant – the equivalent of the major – a major in non-Irish armies can be called maor airm), leifteanant-choirnéal (lieutenant colonel), coirnéal (colonel), briogáidire-ghinearál (brigade general), maor-ghinearál (major general), leifteanant-ghinearál (lieutenant general), and ginearál (general). Colonel-general (a rank in some armies) can be translated as coirnéal-ghinearál, and a field-marshal is marascal machaire with a nice allitteration.
A civil war is called cogadh cathartha, a rebellion can be reibiliún, but also ceannairc. Reibiliún is a masculine word, ceannairc is a feminine, with the genitive form ceannairce. The verb “to rebel” is éirigh amach/éirí amach: D’éirigh muintir na tíre amach in aghaidh an rialtais “the people of the country rebelled against the government”. Éirí amach as a noun is masculine, and can be used of a popular rising, such as Éirí Amach na Cásca “the Easter Rising”. Another possible phrase is chuaigh muintir na tíre chun ceannairce ar an rialtas. And note also the expression dul chun cearmansaíochta (ar…). It is not exactly the same as dul chun ceannairce, but it means “get out of hand, become uncontrollable” and of course it can refer not just to unruly children (chuaigh na páistí chun cearmansaíochta ar an múinteoir) but also to certain situations in war: chaith na fórsaí forghabhála leathbhliain ag iarraidh na treallchogaithe a choinneáil faoi smacht, agus bhí ag éirí leo ar dtús, ach sa deireadh chuaigh na treallchogaithe chun cearmansaíochta ar fad “the occupying forces spent half a year trying to control the partisans, and they seemed to be succeeding to start with, but in the end the partisans got out of hand entirely”.
Partisan warfare or guerrilla warfare is treallchogaíocht, and I see no reason why we couldn’t use treallchogaí for a partisan or a guerrillero. The new online dictionary gives also the words pairtiseán (a partisan), guairille (guerilla), but I mostly prefer treallchogaí, treallchogaíocht. A warrior can be laoch (“hero”) or gaiscíoch, but there is also the word faraire, probably an English loanword. It tends to be confused with fairtheoir though, i.e. a sentry. For “Halt! Who goes there?” I’d recommend Stad! Cé atá ansin?
A tank is tanc, plural tancanna; an armoured car is gluaisteán armúrtha. Troops are usually trúpaí, but note also feadhain (feminine, genitive feadhna) and díorma, the latter meaning something like “troop detachment”. Díorma is masculine and has the plural díormaí.