My niece wants an Irish Gaelic translation of “through thick and thin.” I forgot to ask why, but I suspect it’s for a wedding band inscription. I don’t know if the phrase would translate idiomatically. If not, a sentiment with a similar connotation might do.
Maybe this will help -
Nil me as, ach,
Ask these guys, they love stuff like that: The Daltaí Boards: General Discussion (Irish and English)
Just a translation of some of the suggestions given on that page:
Le chéile idir caol 's leathan
Together through thick and thin (very literal)
Idir achrann agus áthas
Between fights/difficulty and happiness
tríd an daibhreas agus tríd an saibhreas
Through poverty and wealth
trí mín agus garbh
through the smooth and the rough
Oh, and to add a suggestion of my own:
Trí chora crua an tsaoil
which means literally, through the hard situations of life
Maith thú a Scathach!
This is great. Thanks, folks.
Incidentally, I went looking for some seanfhocal’s (Irish proverbs, literally old words) in case there were any nice marriage ones. The results were …interesting.
Ní féasta go rósta, 'is ní céasta go pósta.
There is no feast like a roast, and no torment like a marriage.
Ná gabh bean gan locht.
Do not take a wife without fault.
Is maith an bhean í ach níor bhain sí a broga di go foill.
She is a good wife, but she hasn’t taken off her shoes yet.
Níl aon leigheas ar an ngrá ach pósadh.
There is no cure for love but marriage.
Folíonn grá gráin.
Love veils ugliness.
Encouraging, no?
I wish I’d used some of them for my friend’s wedding last August. Instead I cobbled together a blessing using the aul’ Focloir Póca. 
I would ask my Irish husband, but he’s out, you guessed it, drinking beer. Who’da guessed! He picked out some lovely music in gaelic for our wedding and I bet he has some good ideas. That’s a great idea for a wedding band engraving! I certainly hope they each have a claddaugh!
Gaelic spelling is SO beyond me. I’m still stuck on “chuckie are law” and why my sister-in-law Chivon spells her name Siobhan.
I think all you need is a suitably reverent tone.
Said with the right air of ponderous depth even “An bhfuil cead agam dul go dtí an leithreas” is going to sound meaningful. 