Becomes tríd before an. (Oops! All these years I have happily written it as tríd even before plural na!) It tends to be permanently lenited (thrí) and you do see the form tríd even before a noun without an article (this is dialectal though).
It’s the usual rules before an article. Before a noun without an article, trí lenites.
Personal forms: tríom, tríot, tríd, tríthi, trínn, tríobh, triothu.
Main meanings of trí:
- through: tríd an bhfuinneog, tríd an doras
- by doing something, by using something, by doing something in a way: rinne sé an t-aireagán trí mhiontaighde “he made the invention by minutious research”
- through the medium of: ag múineadh trí Ghaeilge
This preposition is only ever used in Cork Irish (well, probably sometimes in Kerry before names of holidays) and in legalese. I confess I have no exact idea what it means, but it follows the usual rules when it comes before an article. Before a naked noun, it lenites, with the exception of labial consonants (b, m, p). For main meanings, consult the online Ó Donaill dictionary. – OK, fine, I am pulling your leg. It usually means “about, around”, and it is often used with temporal nouns: um an dtaca so (yes, it is definitely a Munster preposition, so um an eclipses the t’s and the d’s!), um an gCáisc, um an Nollaig. In legalese, it refers to what a law or an act is about: an tAcht um Theascadh na mBod Rófhada “the Too Long Penises Amputating Act”.
It has the personal forms umam, umat, uime, uimpi, umainn, umaibh, umpu. However, one of my readers pointed out on Twitter that she had never seen these forms before. They are hardly ever used anywhere else than in the native literature of Co. Cork; a book where you could expect to see them is Dónall Bán Ó Céileachair’s Scéal mo Bheatha.
Lenites naked nouns, but follows the usual rules with articles. The form roimis an… rather than roimh an… is Munster Irish.
Personal forms: romham, romhat, roimhe, roimpi, romhainn, romhaibh, rompu.
- before (in a temporal sense): roimh an Nollaig, roimh an gCáisc
- before (in an order of preference, arrival, prestige etc.), ahead of: tháinig siad abhaile romhainn
- waiting for someone: nuair a thuirling an t-eitleán i mBaile Átha Cliath, bhí cuid mhór de lucht ár leanúna ansin romhainn
Lenites naked nouns, except in sayings of a general meaning (thar barr, thar muir, thar bord, thar claí, thar smacht, thar sáile); the usual rules apply before a definite article.
Personal forms: tharam, tharat, thairis, thairsti, tharainn, tharaibh, tharstu.
- over, across, to the other side of something: chuaigh Seán thar sáile agus bhunaigh sé gnó ríomhaireachta i San Francisco
- over, above: chuaigh an t-uisce thar an gcloigeann air agus bádh é
- going or getting by or past something: chuaigh siad tharainn agus an choiscéim ghasta sin fúthu
- beyond: chuaigh na páistí thar smacht ar an múinteoir bocht
- in preference to: roghnaigh mé an ceann maith thar an drochcheann
I have always had this idea that when it means “between”, it does not lenite the following noun, while when it means “among”, it does. However, Ó Donaill tells us that it basically lenites, with the exception of certain phrases. Whatever. It does not affect a noun preceded by article, so no “usual rules” there.
Eadar is a common variant spelling, typical of Ulster writers.
It has personal forms only in plural: eadrainn, eadraibh, eatarthu. These are only used alone. If idir is followed by two pronouns, those are kept: idir sinn agus iad, idir sibh agus sinn, and so on. There is one book – An Fhiannuidheacht by Cormac Ó Cadhlaigh – where you see stuff like eadrainn agus iad rather than idir sinn agus iad. The book is otherwise written in a rather commonplace Munster Irish for the most part, so I don’t know whether this is an archaism or a hypercorrection.
- both (…and)