My Lagan Love (Traditional Irish folk song).
Marianne Lihannah sings and plays verse 1 and 4 of the beautiful and mysterious, 'My Lagan Love', over Nyckelharpa drones, with a little nyckelharpa solo in the middle.
words Joseph Campbell (a.k.a Seosamh MacCathmhaoil, 1879-1944)
tune traditional Irish (the Belfast maid).

The Lagan river in this song, is the Lagan near Belfast. Campbell was born in Castlereagh Road, east Belfast, just a couple of miles from the Lagan.
In his youth he suffered from some "nervous" problems and often went for long rambles along the Lagan's banks.

From Wikipedia:
"My Lagan Love" is a song to a traditional Irish air collected in 1903 in northern Donegal.
The English lyrics have been credited to Joseph Campbell (1879–1944, also known as Seosamh MacCathmhaoil and Joseph McCahill, among others). Campbell was a Belfast man whose grandparents came from the Irish-speaking area of Flurrybridge, South Armagh. He started collecting songs in County Antrim. In 1904 he began a collaboration with composer Herbert Hughes. Together, they collected traditional airs from the remote parts of County Donegal.

While on holidays in Donegal, Hughes had learned the air from Proinseas mac Suibhne, who had learned it from his father Seaghan mac Suibhne, who in turn had learned it fifty years previously from a man working with the Ordnance Survey of Ireland. Campbell said that mac Suibhne knew the tune under the title of "The Belfast Maid", but did not know the words. A song by this title was published in various early 19th century broadsides, with the first lines "In Belfast town of high renown / There lives a comely maid".This ballad now has Roud number 2930.

The Lagan referred to in the title most likely is the River Lagan in Belfast. Campbell's words mention Lambeg, which is just outside the city. The Lagan is the river that runs through Belfast. However, some argue that the Lagan in the song refers to a stream that empties into Lough Swilly in County Donegal, not far from where Herbert Hughes collected the song.