Of the female vocalists who sang with De Dannan the most likely one to have sung 'as gaeilge' was Dolores Keane. The main song I've heard her sing as Gaeilge would have been Moll Dubh an Ghleanna, but I'm not certain that she sang this on a De Dannan record. If this is the song you thinking of I can probably dig out the lyrics and post them, if you think this is the one you're thinking of.

The Chorus goes something like:

Sí Moll Dubh a' Ghleanna, Sí Moll Dubh an Earraigh,
Sí Moll dubh is deirge ná na Rósa,
Is dá Bfhaighfainnse féin mo romha, de mhná óga an Ghleann,
Sí mo Mholl dubh a' ghleanna a bhfearr liom

Does this sound familiar?