In the video posted online, the thief runs away from the yard of the Carry Out off-licence in Nenagh, Co Tipperary, in fear of Pat and his umbrella, and making his getaway over a wall.
Mr Keller, who is also a local funeral director, said his reaction was in the heat of the moment.
“One of the tenants in the building above my Carry Out rang me to say there was someone trying to break into the cage [where the kegs are kept],” said Mr Keller.
“I was on my way to a funeral and had to fly over there in the hearse. When I got there, the only thing I could find to defend myself was a Keller’s Funeral Directors umbrella in the hearse.”
Mr Keller said the thief had been banging on the lock with a hammer for a good 10 minutes before he arrived.
“I went up to him and asked him: ‘Hi, what’s your story,’ and thankfully he put away his hammer. He then mumbled something at me and as he tried to get away I gave him a crack of the umbrella,” he said.
“He managed to get over the wall, but took a good fall over it as he tried to get away.”
After telling his friend James Williams about the incident, Mr Keller and Mr Williams decided to add the video to Facebook, YouTube, and the website Mr Williams runs, thefinest.ie.
Pat says the video paid dividends almost immediately. “Within a half an hour of putting the video up on Facebook I had the nickname of the lad that tried to rob me and an hour or so later I had his name, which I passed onto the guards.