"Only two drinks tonight" joke

I heard this joke recently from Martin Black, an Irish folksinger who tours
with his two brothers as "The Black Brothers." In the Irish idiom, it went
thus:
An Irish walks into a bar in Dublin, orders three pints of Guiness and sits
in the back of the room, drinking a sip out of each one in turn. When he
finishes them, he comes back to the bar and orders three more. The
bartender asks him, "You know, a pint goes flat after I draw it; it would
taste better if you bought one at a time." The Irishman replies, "Well,
you see, I have two brothers. One is in America, the other in Australia,
and I'm here in Dublin. When we all left home, we promised that we'd drink
this way to remember the days when we drank together." The bartender
admits that this is a nice custom, and leaves it there.
The Irishman becomes a regular in the bar, and always drinks the same way:
He orders three pints and drinks them in turn. One day, he comes in an
orders two pints. All the other regulars notice and fall silent. Whne he
comes back to the bar for the second round, the bartender says, "I don't
want to intrude on your grief, but I wanted to offer my condolences on your
great loss." The Irishman looks confused for a moment, then a light dawns
in his eye and he laughes. "Oh, no," he says, "everyone's fine. I'm just
off the liquor."