The Irish actor knew nothing about the disorder, also known as happy puppet syndrome, when James was first diagnosed - and now understands why his son is so happy when he's in or near water.
In a rare interview about his son's condition with U.S. TV host Ellen De Generes, which aired on Friday (13May11), Farrell said, "James is kind of obsessed with water... He's not a great swimmer but he just loves being close to it and in and around it. It's one of the characteristics of the condition that James has.
"So if James, like, gets upset at home I can get a pot and put some water in it... It's really beautiful 'cos water is such a pure element... There's something really profound about it."
Doctors initially misdiagnosed Farrell's son with cerebral palsy, but his love for water made one medical expert take a closer look.
The actor recalls, "The doctor saw him one day and said, 'Is he always this happy... and does he always kind of flap his arms like that?' and we said, 'Yes'. And he said, 'And how does he feel about water?' and we said, 'Oh, water's fascinating (to him)'.
"He went, 'Has he been tested for Angelman Syndrome?' We didn't know what that was."
The Miami Vice star revealed that, by speaking about his son's rare condition, he has helped the parents of other children with Angelman syndrome.
He explained, "When I spoke about it first about, which was about five years ago, a lot of people got their children correctly diagnosed as a result of just this little interview I did with an Irish newspaper."