The final mission with Irish takes place immediately after completing the raid on Fort Mercer, as Irish attempts to take you across the river to Mexico. Of course, in true fashion, it doesn’t go according to plan and you have to make your way downstream instead of across!
There’s an easy to miss long-range Springfield Rifle laying on the boxes of the raft and an unlimited stash of rifle ammo in a box nearby. You’ll encounter a lot of strong resistance as you head down the river, so stay in cover and use your newly found rifle to pick off enemies far away.
It’s a slow reload though, so be wary if you need to reload near a large number of enemies. Also, watch out for dynamite throwers, they’ll always have a box of TNT right beside them, making them rather handy in disposing of the enemy. Near the end of the run, you’ll spot a couple of bandits - conveniently - on horses!
Shoot them from a distance and when you reach the shore, you and Irish will nab them and part ways. Welcome to Mexico!