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1.
The act of having sex with your own cousin.
Jacob Conklin said "Fuck the cousin rule" and performed an irish hand grenade.
by The_Notorious_Dr_J September 05, 2008
 
2.
Molotov Cocktail or other Improvised Explosive Device (IED)often used by Irish Nationalists like the Irish Republican Army (IRA).
An Irish Hand Grenade is typically made of inexpensive and available materials, such as glass bottle that breaks upon impact allowing flamable liquids inside to ignite from burning rag sticking out of end of bottle.
by MacanUltaigh January 17, 2009
 
3.
While consamating with a women u bust or jizz in your hand and scream "FIRE IN THE HOLE" and then throw the handfull of jizz which looks like a grenade into her face therefore EXPLODING into her eyes
"Dude i nailed Tracy in the face with an Irish Hand Grenade last night"
by Matt Friedman July 30, 2006