Northern Irish Cardio, is what happens when a member of The Irish Republican Army (I.R.A.) throws a molotov cocktail/hand grenade into a protestant church, british army base, or the house of someone who is giving the english information. then running away as soon the building busts into flames or explodes.
Murphy: Hey Sullivan where ya going lad?
Sullivan: Down the road to Kelly's flat, for Northern Irish Cardio.
Murphy: Northern Irish Cardio eh? must've been talking to the brits then.