This is a retelling in me own words:
Seamus, Sean, and Derrick would always go to the pub of a Saturday evening. They would each buy a round of drinks in turn, and they would drink to each other's health and good fortune, and they would drink to each other's families and future happiness and successes. The bar tender knew that he should keep those drinks a coming three at a time. The tradtition was set, and it was important that they never miss the chance to be together as brothers, to talk about football, naturally, and to talk about work, their women, and the wee ones.
So as it happened, the brothers made a pact that no matter where they lived, they would always pour up the drinks by threes and wish one another all the best. If they be separated by holidays, by work, or even by death, they would certainly be together for their Saturday night toasts.
Well, sure enough, the time came where they must separate. Seamus married an American girl and left his dear Ireland to live in Chicago. Sean likewise moved to the States and took a job in New York. So Derrick alone stayed behind in Ireland. Yet, true to their promise, Saturday nights would find each of these lads in a small Irish pub reminiscing and wishing hard for the welfare of his brothers. They would write as often as they could, but never often enough. But the memories of all the times together would flood back to mind sure each Saturday evening as each brother sat down to a round of drinks.
The bartender in the Irish pub got used to Derrick's strange ways. At first he thought it was strange that he'd order three drinks at a time. So he told him that the draft would last longer if he'd just get one at a time. But then he learned of the pact the brothers had made, because Derrick explained all to him. So each Saturday night, the bartender would pour up the drinks three at a time, just as he always had in days gone by. And Derrick was always as happy and chipper as could be. After two or three rounds, he in fact was talking to his brothers just as if they were truly there.
So the bartender in the Irish pub had been observing Derrick for months now. He didn't want to intrude on this new melancholy; Derrick seemed so solemn and reflective. For two weeks now Derrick was only buying the drinks two at a time. Of course, the bartender knew that something had happened to either Seamus or Sean. He loved them dearly himself, and he was afeared to hear the truth of the matter. Strumming up all the courage he had, he offered his deep condolences.
"Derrick, I'd like to tell you how sorry I am that you've lost your brother."
"How's that?" says Derrick, "Me brothers are both fine. Sure you know that they are in America now."
"Yes, that I realize. It's just that you are only buying two drinks at a time now, and not the regular three."
"Aye, so I am," said Derrick, just as matter-of-factly as you can imagine. "Both my brothers are in the best of health, and their families are well indeed. You see, I only buy two drinks at a time these days, because Thursday evening a fortnight back I joined the Mormon church, so I did."
(So being a good Latter-day Saint myself, I would hope that our missionaries do a better job teaching. But being half-Irish myself, I couldn't miss the humor of a great and silly joke. I know of no other people who tell so many jokes on themselves.)