I agree with cleod, Donal agus Morag is the best song for a wedding - if you speak Irish gaelic. Altan do a brilliant version on their album Harvest Storm and they also give the words and translations.

I sang a song at my friends wedding which went down very well. It's called Dark Inniseoghan. Its about a soldier who searches for his true love and finally finds her in the penultimate verse. The air is stunning. The Northern Irish band Deanta do a very good version of the song - the chords are quite easy so there should be no need to find the music score.

Slan go foill

Laoise