The phrase "get your Irish up" is normally used in reference to a fight. With that being said, there's usually a Belfast Brawler at most bars come St. Patty's Day.
These guys simply love to fight. Don't bump into them. Don't look at them. Don't look at any of their friends. These dudes are usually jacked to the gills, have been slamming shots of Jameson all day and can't help but eye everyone in the bar as if he's Rocky and they're Ivan Drago.
Bumping into the Belfast Brawler is like running into the eye of a tornado. When dealing with this ape, remember one all too important word -- avoid.