The mercenaries.
The Irish mercenaries were sent by Wallenquist to hunt down Dwight McCarthy and bring evidence for the death of Jack Rafferty in order to ferment a war between the Old Town and the Police. The group are presumably former members of the IRA who have fled to the United States.
When Dwight went to The Pits to dispose the bodies, he was shot by the mercenaries but he survives only because Jackie Boy's badge blocked a bullet, as Murphy discovers moments before being shot. Dwight kills the assassins, but is attacked by more of them and a grenade (from Brian) blows him and the car into the tar pits while Ronnie and Brian cut off Jackie Boy's head.
As Dwight sinks into the tar Miho and Dallas kill those remaining except one. They save Dwight who then tortures the survivor for information. They manage to catch up with the Irish and run into their car. Ronnie with his Uzi shoots Dallas and in return has his head sliced in half by one of Miho's throwing stars.
Brian enters a sewer where he is followed by Dwight and taunts him saying that his gun is wet, yet his grenades and remote are working. Moments before stabbing Dwight, Miho comes and impales him painfully with her katana.