The Irish star has two sons - James, seven, and Henry, nearly two - and spoke about his eldest son's condition while on the Late Show with David Letterman.
He said: 'James, just a quick shout out if I can, James has a condition called Angelman Syndrome and it only affects one in 30,000 children, they say.
'So there's not much funding because it doesn't benefit pharmaceutical companies, of course.'
Speaking about his diagnosis, he added: 'It was actually when a doctor saw how happy he was. He was like: 'Does he always smile this much?''