Hi, Graham - I think the official name of Tom Lehrer's classic is The Irish Ballad (click).

The chords in the Rise Up Singing songbook aren't always the best, but here's what they have:

Dm - DmGm Dm / Gm Dm DmC Dm /
DmGm DmGm / DmC Dm C / DmC Dm

-Joe Offer-