This is a Brodie Gang mission.
"Brodie's gang is falling apart. There are three hardcore members that will never quit, though - Brodie's cousins Sean, James, and Bill. They invited you to a meeting at the scrapyard. You know that it's a trap. What the Brodies don't know, however, is that you are the trapper."
For this one you'll want to be in full health and well stocked on ammo before you begin. When you're ready, start the mission and follow your GPS to Bruski's Scrapyard where the three cousins will hiding out with several of their Irish thugs. As you round the corner you'll see a roadblock ahead. Crash through the chain link gate to the left and hop out, then start picking off the thugs using your car as cover.
The thing to keep in mind here is, until the last of the cousins are dead the thugs will continue to respawn as you kill them, often from behind you. Take out as few as necessary while you work your way to the cousins, who are represented as markers on your mini-map. Keep moving but stay in cover whenever possible; there are plenty of places around to use.
There's no set pattern here; they will be in different places on different playthroughs so just continue to work your way through the junkyard until the last one is dead, then head out to the safe zone to end the mission.