"Tim Shandy stepped into the" joke

Tim Shandy stepped into the Warm Spoon, a popular Galway tavern.
To Mike Callahan, the barkeep, Shandy said "Mike, I'll be havin' three
whiskeys."
Callahan set up three glasses and began to pour. "Now, Timothy, it's not
the usual thing for you to ask for three whiskeys. It's celebratin', you
are.
"Ahh, ye know me too well, Micheal, ye do. Truth, and I'm
celebrating me first blow job."
Callahan smiled benevolently and set a fourth glass on the bar. "Now,
that's special," he said. "For an old customer like y'rself, here's a
fourth on the house, so I may be sharin' your celebration with you."
Shandy shook his head, and replied "'Tis verra kind of ye, Micheal,
but I'm thinkin' if three won't get rid of the taste, four won't either."