The drunken charlatan's icon only appears when you've completed Dickens 'Liars, Cheats and Other Proud Americans' . Ride or warp to Armadillo and you'll find Irish being 'entertained' by a couple of his mates.
When then cut-scene's over, you'll need to fill 'em full of hot lead, so switch to Dead Eye and target both of their noggin's for a swift justice. Now follow Irish on your horse until you reach Lake Don Julio where you need to clear out the area and the house down below.
Sneak down and use your rifle to pick everyone off from mid-range and from behind the safety of cover. When you get to the front of your house, have your pistol or shotgun equipped as you'll be greeted with a surprise as you try and open the front door (so kill him quickly and grab his Double Barrelled Shotgun from his corpse) .
By the time you realise it was all a trap Irish will have sauntered off elsewhere. So it's time to catch up with the swindler, isn't it?