My demographics are a little bit strange here...I asked a lot of people under 25, and those results were scary (didnt specify FOLK song you see)
Under 25 year olds either
A)didnt know one at all (6) or
B)mentioned the group House of Pain (3)
oh..and 1 16 yr old girl said "any kind of Jig"
Among the over 25 set I got the following responses:
A)When Irish Eyes are Smiling (3)
B)Danny Boy(2)
and I had one vote for a song by a Scots band called the Procalimers (I think it's called "500 Miles"?)
at any rate--I had an interesting time asking--sorry the results were so weird!