The Irish heartthrob gave a very revealing answer this week live on TV.
Farrell appeared on the Ellen Show and was given the chance to raise some money for a worthy cause.
All he had to do was tell the audience something he had never told anyone before and Ellen would decide if it was worth up to $10,000.
The actor promised: “This has got to get to ten,” before shocking the audience with a story that explains why he didn’t get a full-frontal scene in a movie.
Just recently his Beguiled co-star Kirsten Dunst revealed how upsetting her sex scene was with Farrel but the actor’s own confession was far more humouros.
He said: “I was doing my first American film I think in the year 2000, and there was a sex scene in it.
“I was very nervous because I’d never done a sex scene – now they’re just a walk in the park – but anyway, I went up to the director, Joel Schumacher, and I had a dressing gown on and nothing underneath…”
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“I was obviously so nervous that I lost my mind for a second. I opened my dressing gown and said, ‘It’s a bit…’ I had what you’d refer to as ‘retro bush.’
“Joel, the director, kind of went, ‘Eww,’ so I said, ‘Ok, I’ll trim it.”
Things get even worse from there.
Farrell continued: “I went into the hair and makeup department – it was their lucky day – and I said, ‘Do you have a beard trimmer?’ And they said ‘Yes’.
“I don’t know if they ever used it again. I got a bin… I sat over the bin and I was nervous and I trimmed the right side a little bit and I obviously had to match it so I trimmed the left.
“But I overshot the mark a little bit, so I had to get back in and approach the right again.”
By this time the audience was in hysterics but the actor still wasn’t finished.
Farrell said: “I overshot that and by this stage I was sweating and I went back in and reapproached the landing on the left and then I looked down, and I had a little landing strip.
“Lovely, lovely. I didn’t get my full frontal, after all.”
Ellen, of course, awarded the shameless star the full amount.
Farrell said the money would go to the CHIME Institute in California. His own son, James, attends the school which mixes children with special learning requirements into regular classes.