The 48-year-old actor became a father for the first time in 2003 when he welcomed James with model Kim Bordenave, who was diagnosed with Angelman syndrome - a rare genetic disorder which involves delayed development.
The diagnosis was enough for Farrell to put his wild ways behind him and sober up for the sake of his little boy.
In an interview with PEOPLE, Farrell said "[I'm most grateful for] my sobriety, everything is magnified when you have a child with special needs, you're even more aware that your child really needs you around and is going to need a lot of help and a lot of support.
"So James was about two when I got clean, when I got sober and he was a big big part of me putting the bottle down, a big part of it.
"Because I was in no condition to be a friend never mind the father of a child with such exacting needs. So if it wasn't for my sobriety, I wouldn't be able to be there for James and enjoy in the marvels of his life and support him in the way that I feel that can ... "
Farrell - who also has a younger son Henry with actress Alicja Bachleda-Curus - added: "I don't want to make out like I"m the perfect dad, I f*** up left, right and centre but at least you have to be present to f*** up so I'm around, but yeah they're very conjoined - my sobriety and my children ... "
The actor revealed he's starting a charity to help young adults with intellectual disabilities as James prepares to celebrate his 21st birthday in October.
"Once your child turns 21, they're kind of on their own," he explained.
"All the safeguards that are put in place, special ed (education) classes, that all goes away, so you're left with a young adult who should be an integrated part of our modern society and more often than not is left behind."
The Colin Farrell Foundation will help young adults with intellectual disabilities through advocacy, education and innovative programs.
Farrell went on to reveal James lives at home with him with the help of full-time carers and he is a "happy young man".
He added: "We hang out, we play ball, swim in the pool, he loves movies ... go out and get a cheeseburger ... He spends a lot of time with his partner in crime, his carer ...
"I've had live-in care so I can move freely and work and it's been great for James as it gives him that autonomy ... He has a good life, he's a happy young man... I'm proud of him every day."
with AAP.
