From the groaners listserv.

A secret agent was sent to Ireland to pick up some very sensitive
information from an agent called Murphy. His instructions were to
walk around town using a code phrase until he met his fellow agent.
He found himself on a desolate country road and finally ran into a
farmer.

“Hello, said the agent, “I’m looking for a man called Murphy.”

“Well you’re in luck,” said the farmer, “as it happens, there’s a
village right over the hill, where there’s a butcher called Murphy,
the baker is named Murphy, three widows are called Murphy. In fact my
name is Murphy.”

“Aha,” thought the agent, “here’s my man.” So he whispered the secret
code. “The sun is shining… the grass is growing… the cows are
ready for milking.”

“Oh,” said the farmer, “you’re looking for Murphy the spy – he’s in
the village over the other direction.”