This is a Brodie Gang mission.
"Brodie just bought new cars for his top boys. He's feeling the heat, trying to buy his gang's loyalty before you turn up for a showdown. He didn't show you any loyalty when he framed you. Trash the cars and show Brodie's crew that sticking with him is a loser's game."
This is a large battle in the heart of the Irish mob's territory so bring plenty of weapons and ammo. The thugs will come at you from all directions, and it's highly likely the police will get involved, making matters even worse.
From the mission start, get in your car and head up the street to the large back-lot where the cars are parked. The best method for this mission is to drive all the way to the back, scoring a few vehicle kills along the way, and park your car at an angle in the far back right corner. This will keep them from flanking you and provide you cover from their attacks. They will come up the stairs to your left, down the alley from the middle and from around the corner to your right. Stay in cover here until you have taken this initial group of thugs out.
Once the first group has been taken care of, break cover and make your way back up the alley blowing up the cars as you go. Near the midway point another group of about four will come up from behind and another from ahead. You should be able to use the corner of the building there as cover to quickly take these groups out.
You most likely had to kill a few cops somewhere in all that carnage so you will have a wanted level. Once the last of the thugs have been killed and the cars all destroyed, hop in any car parked nearby to better go unnoticed by the cops. You should be fine as long as you don't speed so just head down the street to the safe zone to end the mission then go get a change of clothes.