"Drinking Buddies" joke

Two men are sitting next to each other in an Irish-style pub in New York City. They both order pints of Guinness. One of them turns to the other and says "So where are you from, then?"
"I'm from Ireland."
"Me too! I'll drink to that." They both finish their pints and order two more.
"Where in Ireland are you from?"
"Dublin."
"Me too! I'll drink to that." They both finish their pints and order two more.
"Where in Dublin are you from?"
"The East Side."
"The East Side? Me too! What a coincidence! I'll drink to that!" They both finish their pints and order two more.
"Where on the East Side are you from?"
"McDonagh Street."
"Me too! This is incredible! I'll drink to that."
As the bartender pours them another two pints, another customer at the bar says to him, "That's amazing! I can't believe they're from the same street in Dublin. What's going on?"
"Oh, it's nothing amazing," says the bartender,"it's just the Ferguson twins getting sloshed again."