Horse buyer:
"Is this horse for sale?"

Seller:
"That's why he's here."

Buyer:
"How much?"

Seller:
"Fifty pounds."

Buyer:
"Hmmm. Has he any faults?"

Seller:
"He only has two faults, to be honest."

Buyer:
"What are they?"

Seller:
"Now, you can't expect me to tell you them both. But I'll tell you what I'll do -
I'll tell you the one fault now, and the other when you've paid your money."

Buyer:
"Well then, tell me the first fault."

Seller:
"When he's loose in the field he's the very devil to catch."

Buyer:
"Oh, that's not too bad. I'll take him. Here's your money. And now, what's the second fault he's got?"

Seller:
"When you have caught him, he's no good for anything."

From the Appletree Press title: The Male of a Cow: An Anthology of Traditional Irish Humour by David Ross.
Also from Appletree: Never Throw Stones at Your Mother: Irish Insults and Curses.