Grace O'Malley never made it being an old woman, I don't think. If you listened to the Virgin Queen, Grace was a pirate. She took of the chieftancy of her clan after her husband's death, and she raded the English coast to get supplies and money. Granuaile (with the various spellings in Old, Middle and Modern, not to mention Scots) is Gaelic and in English it's translated to Grace.
There was also a mythical Granuaile, and that's the one to whom I think the song is referring. She was, I think a mother/goddess.