I'm sure someone will ask for tomorrow:
How do you say 'Happy Fathers Day' in Irish? I figured I'd preempt it and ask for verification myself!

Is this correct? Lá na nAithreacha Sona Duit!

When I was typing this, I didn't know whether it should be Happy Father's Day or Happy Fathers' Day. Is it about one dad or all of them? That presumably influences the translation and now I have just managed to tie myself up in knots (and not in a good way.....!!)

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On greeting cards, etc. it's usually singular, since it's directed to just one father in particular.

Lá na nAithreacha sona duit sounds right (although it might mean "the day of the happy fathers for you" )

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Is fearr Gaeilg na Gaeltaċta ná Gaeilg ar biṫ eile
Agus is í Gaeilg Ġaoṫ Doḃair is binne

It seems it's Father's Day (erroneously) in English. I'd go with Lá na nAithreacha sona duit too.

http://en.wikipedia.org/wiki/Father's_Day

Although the name of the event is usually understood as a plural possessive (i.e. "day belonging to fathers"), which would under normal English punctuation guidelines be spelled "Fathers' Day," the most common spelling is "Father's Day," as if it were a singular possessive (i.e. "day belonging to Father"). In the United States, Dodd used the "Fathers' Day" spelling on her original petition for the holiday, but the spelling "Father's Day" was already used in 1913 when a bill was introduced to the U.S. Congress as the first attempt to establish the holiday, and it was still spelled the same way when its creator was commended in 2008 by the U.S. Congress.

I've found -
Lá na nAthar faoi shona duit
and - Lá na nAthaireacha sona duit (though I'm assuming that there's a mistake there)

Several things are possible, but "na n-athar" is a bit surprising...

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Is fearr Gaeilg na Gaeltaċta ná Gaeilg ar biṫ eile
Agus is í Gaeilg Ġaoṫ Doḃair is binne