A Frenchman, a Welshman and an Irishman

 is a mission in Red Dead Redemption.

Story
The mission begins with John finding Irish having his head dunked in a horse trough by two men intent on killing him. After killing the two men, John asks Irish if he might know the location of a Gatling Gun needed to assault Fort Mercer. When Irish responds that he knows the location, they travel to it with Irish disappearing at the sight of men guarding the house. After John kills all of the men and heads inside, he finds there is no Gatling Gun to be found, and that Irish has long since disappeared from the area completely.

Walkthrough
This mission requires no explanation. All you have to do is follow Irish to the homestead, kill all the men, and go inside the cabin.

One tip, though: beware of the hidden enemy that jumps out of the cabin when you get near the door. He is sporting a Double-barreled Shotgun which causes a lot of damage.