I have read before (somewhere...I thought here on Mudcat) that the sorrow was TB, which was (so I read) both quite common in Ireland and not spoken about publicly.

It is one of the finest examples of telling a complete story in a truly concise fashion. And if it is TB then I thing the story is complete. A `time line' would then go something like this:

Boy meets girl, they fall in love, plan to marry. Her parents are OK with this idea despite him not being that well off.

They meet at an event of some kind (a fair) and go about their separate business. But this happens to be the last time they meet because she has TB and by the time she gets home she falls ill with it and dies.

He dreams she comes to him (or she does as a ghost) and tells him (`it will not be long love till our wedding day') that he too has TB and will soon die. The word `dream' here might be appropriate actually, as I assume the latter stages of TB might lead to hallucinations and/or delirium.