This is a Brodie Gang mission.
"Brodie is losing his grip on his protection racket. One shopkeeper is refusing to pay the fees. Protect this shopkeeper and his shop, and surely more of Tam's victims will stop paying too. When Tam's income starts drying up, his gang will evaporate."
Head to the back alley where you'll find the shopkeeper cowering beside a large dumpster. It's a pretty basic mission; the Irish thugs will come in groups of three or four from either end of the alley. Take cover along the wall or the end of the dumpster, depending on which direction they come, and take them out.
Though not necessary, one way to make it go a lot easier is to drive your car back there in the beginning of the mission and use it to smash the three small green dumpsters right by the shopkeeper. This will remove the cover they use and also open up your line of fire to the south.
After about eight waves you will be done so head back up to the safe zone to end the mission.