I found it interesting to read MississaugaSean's account as my name is Sean also and I grew up in Mississauga (suburb of Toronto) and I heard this same rhyme bandied about (though not directly at me,from memory). I am now 47 years old, so i heard this back in he 1970s. How strange that there is a folk memory of this verse in the Mississauga area from the 70s/80s. When i first googled 'Sean, Sean, the leprechaun, went to school with nothing on'I assumed it would reveal an internationally recognised rhyme. But maybe it was just invented by someone in Mississauga or Toronto. and it never really spread much further. I now live in Australia, however!

Cheers, Sean