Alan, I used to think that it was a romantic love song from the feminine point of view, but recently I have come to see it as a son being addressed by his father or mother. This is mostly supported by the supposition that the singer will be dead when Danny returns (much more likely of a parent than a sweetheart) and the epithet "boy" (a sweetie would say "dear" or some such). I also find it incongruous that a girlfriend would expect him to meet her in the hereafter, or if it's his wife that she's not going with him.

I think it's the parting words of a parent as the lad leaves the nest to seek his fortune in the new world.