The 37-year-old singer, Ronan Keating, has revealed the embarrassing incident occurred when he met Pope Benedict XVI in 2011.
He recalled: "I was at the Vatican and we had just done this performance and there's a line up to meet the Pope afterwards so we were all standing there and got told all these different protocols of what we're supposed to do.
"I was so mixed up, by the time that I got to him that he got to me, I had no idea what to do, whether I was supposed to kiss his hand or the papal ring.
"I got so confused that I just thought I'll just headbutt his hand."
Ronan said Pope Benedict, who resigned in 2013, didn't make a big issue of of the incident.
The Irish singer told 2DayFm: "It was one of those awkward moments that I'll never forget, he didn't say anything, I don't know if he speaks very good English, he kind of just looked at me and moved on.
"I was thinking the fist pump might have been better.
"You can never take the Dublin out of the fella."