Wasn't "Dirty Old Town" written by Ewan Mc coll for one of the Radio Ballads ?
I would suggest that the young man's name originates in the Irish "Muiris" + the diminutive suffix "ín" thus giving "Muirsín" spelled in various ways by those with no knowledge of ther language as Mursheen , Murshin , Murshen etc.
Annraoi