Just came across this thread as I was trying to find the song where I'd heard of Tir na nOg. (The reason for this is that my wife and I have just booked a night's stay at a B&B near Cashel that's called Tir na nOg. We're doing a two week self-drive tour of Ireland next month.) I wanted to know what the translation is into (dare I say it?) English.

The song, of course was "Uncle Nobby's Steamboat." I think I found it on a (borrowed) LP by The Wolfe Tones which I no longer have. But the jacket had the words which I copied down, including the 4th verse referred to by Wolfgang, and printed with the following spelling:

4.We're off to Tir na nOg, the goddess of love is Grainne
Cuchulain can't get in, in his coat he has no fainne.
The Leprechauns go by in a twinklin' mystic vision
The shamrock lies a-mournin' in the grave.

I don't have the Gaelic but the pronunciation by the singer(s) is thus:
Tir na nOg = teer nan og
Grainne = Gran'-ya
fainne = fahn'-ya
Cuchulan = kook-lon

If anyone knows the background of this song, and the meaning or intent of some of the words or phrases and what they refer to, please post the info here. (Especially the translation of the Gaelic words!) Thanks.