You're a manager. Of a restaurant. Which apparently has a bar. And it's St. Paddy's day. . . . .
What exactly did you expect?!
The day off because you're of Irish lineage?
Look, I feel your pain---nothing sucks more than having to work on awesome holidays (when I was a bagboy for a grocery store in middle school, I had to work on the 4th of July!)...but I'm thinking it has less to do with him being Italian, or even his douchiness, and more having to do with the fact that his restaurant will no doubt be slammed on St. Paddy's, and he probably needs his manager there to deal with the insanity.
Enjoy those jamison's and stouts---and cheers
from a fellow Irishman.