(n.) The predictably jagged top half of a recently broken glass bottle held at the neck by the wielder.
Mickey called Paddy's sister a whore from Revere so Lenny grabbed a bottle of Jameson; made an 'Irish knife' on the bar; and held it under Mickey's nose and made him apologize. Then they all made their peace and went out to beat up some smart kids from Harvard.
A potato peeler.
Bobby O'Connor whipped out his Irish knife to defend himself last night while he was being robbed, and this morning, he peeled potatoes with it.