Augusta Gregory's translation is clearer:

O Donal Oge, if you go across the sea,
Bring myself with you, and do not forget it;
And you will have a sweetheart for fair days and market days,
And the daughter of the King of Greece beside you at night.

So the speaker is promising that she herself will be as good as the daughter of the King of Greece to Donal, an extravagant promise, but sure you never know.