I'm pondering what gives a song [or tune] it's nationality.

Lots've folk fests I go each season have events billed as 'Irish session', 'English session' etc but I often hear other musics played which haven't originated in those nations.

A well known example is 'Clare to here' [R. McTell] often assumed to be an Irish song.

Its a Irish setting, but McTell is English, so what's the song's nationality?