One would have been forgiven for thinking that every facet of the Dubliner's life was open season for our edification. Yet there was one subject so close to his heart that he couldn't bring himself to talk about it publicly for years -- Angelman Syndrome, the little-known, severely debilitating neuro-genetic condition suffered by his son, James.
In fact he hid this life-changing detail from everyone but his family and closest friends to protect his boy.
But now, the In Bruges star regrets not talking about the subject from the off.
Speaking as guest of honour at the Foundation for Angelman Syndrome Therapeutics (FAST) gala in Chicago at the weekend, the actor opened his heart about life with eight-year-old Jimmy, as he affectionately calls him, and how the happy-go-lucky child -- who like most other AS sufferers has speech and movement impairments -- has greatly improved the quality of his life.
"He's kind of lifted me in that sometimes tremulous hand of his," he told the delegates. "I ain't saying I'm a great man or nothing, but I'm getting to be about as good a fellow as I'm going to be.
"A lot of the reason is James has come into my life and gifted me the opportunity to look outside myself and to watch his every difficulty and to watch his insurmountable spirit just forge ahead in life."
A journalist asked him why he was involved with the organisation and, determined to protect James's privacy, he gave a pat answer.
"I found myself not speaking about what I wanted to really speak about so much," he said at the weekend.
That night, he said he went to bed devastated that he hadn't been honest: "I just thought, 'Well, maybe it's time. I just have to speak about him. And once I'm not protecting him, what's going to happen if I speak publicly about his condition? Nothing really."
He called his son's mother, model Kim Bordenave, and suggested the time had come for him to acknowledge James's condition. The next day he spoke about Angelman for the first time during an interview. Since then, he has helped raise awareness by talking about James's condition whenever he is on promotional rounds.
An appearance on the Letterman chat show in America led to a significant rise in donations to FAST and turned Farrell into something of a poster boy for AS. And, such has been Colin Farrell's enthusiasm for the charity's work, he sponsored 100 families which enabled them to attend last weekend's conference and share their experiences.
The syndrome was discovered in 1965 by Dr Harry Angelman, and initially known by the now-pejorative name, Happy Puppet Syndrome -- a reference to the smiling demeanour of many of those with the condition.
Tests for AS only became widely available in the mid-1990s.
Sarah Roarty, a chemist based in Co Galway, attended the Chicago gala. Colin Farrell's dedication towards raising awareness impressed her.
"He was fantastic -- a really lovely guy," she says. "Anything that helps people to learn more about Angelman Syndrome is a good thing."
Sarah's nine-year-old daughter, Clara, has the condition and has severe intellectual disabilities.
She has an extremely limited vocabulary although her communication skills have improved with the help of Lámh -- a rudimentary form of sign language for the profoundly disabled. "And we can thank Steve Jobs too," Sarah says. "The iPad is a great tool for children with Angelman to communicate."
Clara was 18 months old when she was diagnosed. "I'd never heard of Angelman before," Sarah says.
"It was like a bolt from the blue. For the first year we thought Clara was
a normal, healthy child. Then we found out that she has a rare, debilitating condition and our world was turned upside down.
"Initially, we found it very isolating, because so few people have it but we've learned to cope as the years have gone by, and social media sites like Facebook have made an enormous difference in terms of dissemination of information and communication with other families."
Sara Hetherington's five-year-old daughter, Orla, also has Angelman Syndrome. "It's thought that the condition affects one in 20,000 people," says Dubliner Sara.
"We know of 50 to 60 families in Ireland who have a member with it, but the actual number is probably in the region of 100."
Sara's website www.angelmanireland.com details her experiences as a full time carer of an AS sufferer.
"When you're told your child has a rare condition like this, your natural instinct is to try to reach out to others who are in the same position," she says. "There's a certain comfort in knowing that you're not alone."
Meanwhile, Sarah Roarty is greatly enthused about a recent breakthrough which could yet improve the life of Clara and all those who suffer from AS.
"While investigating Alzheimer's disease, researchers succeeded in restoring the intellectual, cognitive and motor functioning capabilities of a mouse with Angelman Syndrome. "This suggests that a treatment for the effects of Angelman Syndrome in humans is possible yet frustratingly remains out of reach due to lack of funding."
Sarah is also concerned about how austerity measures have hit the most vulnerable in society. "The Government is thinking in the short-term when it comes to disability, and not long-term as they should. If [Social Protection Minister] Joan Burton spent the day with a child with AS she might not be as ready to make cuts."
Despite the challenges of having a child with Angelman Syndrome, Colin Farrell knows that his wealth ensures that, unlike many Irish families, he will never be affected by proposed cuts in disability allowances.
"Jesus, I'm really lucky," he said earlier this year, "because [having money] allows me to pay for the physio and the speech therapists Jimmy needs.
"I can pay for really good ones, the best. I'm afraid a lot of people whose children have conditions really struggle."
WHAT IS ANGELMAN SYNDROME?
- Most children with AS have microcephaly (small head size) and epilepsy (recurrent seizures). The developmental delay of AS generally becomes noticeable by the age of six to 12 months.
- Some of the characteristic features of AS are a result from the loss of function of a gene called UBE3A.
- The life expectancy for individuals with AS appears to be nearly normal.
If you would like make a donation to aid research into Angelman Syndrome, visit www.angelmansyndrome.ie