This is from a review of a record by the Battlefield Band which features a recording of the song:

"The final song, The Mickey Dam, illustrates one of the simplest. The construction of Milngavie waterworks in the late 1800’s brought vast numbers of Irish labourers to the area, just north of Glasgow. Conditions were harsh and, as so often happened, triggered the inimitable Irish wit. This resulting song has been sung by Glasgow’s Irish community ever since."

Looking at the entry in the DT, it needs some corrections! 'Laborer' is the American spelling, so should be 'Labourer'. Also, the place name mentioned is Milngavie, not Mull Guy (Milngavie is pronounced more like 'Mull Guy', to the consternation of many non-Scots).