Joseph Campbell (Seosamh MacCathmhaoil) was a North Irish song collector (1879-1944) with Irish speaking grandparents. His partner composer Herbert Hughes heard this song first in County Donegal (by someone who knew it from his own father, who in turns learned it half a century before). They've got their own river Lagan there (spilling into Lough Swilly), different from the Belfast Lagan.
Margaret Barry [Alan Lomax recording]
Lonnie Donegan [as My Laggan Love]
Pete Seeger [the first one as The Quiet Joys Of Brotherhood; later version with own words as Pete's Song]
Richard & Mimi Farina [idem; Richard wrote these lyrics]
Fairport Convention [idem]
Sandy Denny [idem]
Kate Bush [B-side of Cloudbursting]
Simple Minds [melody under outro Belfast Child]
Unthanks [with Brighouse And Rastrick Brass Band; vocal: Niopha Keegan]
Which Lagan this song is about, the river that leads through Belfast or the smaller Lagan flowing through east Donegal and west Tyrone, is irrelevant. According to Sinéad O'Connor it's about Ireland as a whole. The song was written in a time when it was against the law to write favorably about Ireland. So poets and writers referred to the land as if it was a beautiful woman. There's a Lagan in both Ulster and the Irish Republic, that alone makes it political.