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2.
A hangover, usually involving whiskey; may be mild or severe
"I’ve still got a touch of the Irish flu."

"The Irish flu?"

"Also known as a hangover."

(from "A Nose for Murder" by Lee Charles Kelley
by Lee Charles Kelley April 06, 2005
 
1.
A particularly unpleasant hangover.
"Last night was great, but I have the worst case of the irish flu today."
by Melissa July 10, 2004
 
3.
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