Sweeney's Men, from the 1960s, are the kind of group that rock critics would call 'seminal'. Member Andy Irvine later formed Planxty, who were huge in Ireland in the 70s but little known here; and Terry Woods was later part of the first line-up of Steeleye Span, and eventually became the banjo player in the Pogues.
Here are Sweeney's Men with the sad, but comical, tale of the Old Maid In The Garrett...