I am an English born & bred Hassey.My understanding has always been that this name is a French derivative, particularly from Brittany.
However, I was once contacted by another UK Hassey (William) doing Family Tree research who believed that his branch did indeed come from Ireland, and the Waterford area was mentioned (I think).His E Mail is [email protected]
We couldn't establish any connection between us, so who knows.Other derivatives of the name seem to be Hussey and Hasey.
Good Luck to all!!