The ailment one suffers on March 18th after properlyly celebrating St. Patrick's Day. Anyone of Irish descent may also use the term to denote a hangover on any day of the year.
Seamus: Ugh, I was out drinking Guinness until the wee hours. I think I'll call in with the Irish Flu today.
by robbie vienna March 17, 2009
A particularly unpleasant hangover.
"Last night was great, but I have the worst case of the irish flu today."
by Melissa July 10, 2004