The 35-year-old singer was in a backstage bar at the Isle of Wight Festival on Sunday when the fan lifted him up in the air in front of his friends, reported Sun online.
Security guards later asked the fan to leave the VIP area.
"The man was showing off to his mates after they were laughing at Ronan for being small. He always assumed Ronan was a lot taller and caught him unawares by lifting him up in the air for a prank.
"Not surprisingly Ronan wasn't happy at first and had a few stern words. After the punter was carted off by security, he saw the funny side. He always thought he was average height," a source said.
Keating's spokesman also confirmed the incident saying, "Ronan was accosted by a man who was very drunk at a bar backstage. The guy picked him up and he was removed from the area by security."