Definitely the Belfast Lagan:

"Her father sails a running barge
Twixt Lambeg and the Drum"

This places it firmly on the Lagan Navigation from Befast to Lough Neagh. The Drum is from an Irish word meaning a hill, not a reference to the dreaded Lambeg Drum of July 12th fame!

I've seen a reference to the composition of the English words: apparently a couple of fiddlers met while walking near Belfast, and, lamenting that such a fine tune lacked a song, sat down together and wrote one. I think I got that from Paddy Tunney's "The Stone Fiddle".
Incidentally, the song goes well into French: (sorry, no accents)

Ou Lagan fleuve chant ses berceuses
Ma jolie rose demurre.
La crepuscule est dans ses yeux,
La nuit sur ses cheveux.

Et comme une fee, elle m'a attrape,
Mon coeur elle tient toujours.
Elle a ma vie, ma liberte,
La reine de mon amour.