The 34 year-old plays an ex-criminal in the gangster flick London Boulevard alongside Keira Knightley.
He told Daybreak: 'I had a beautiful, smart and witty dialect coach. She came over to Los Angeles where I live most of the time and we sat out in the back garden for about three weeks just going over the script. She was brilliant.'
Farrell said he is much more at ease with the American accent.
He added: 'I grew up watching American television more than I grew up watching Irish television, and more than I grew up watching English TV.
'So by the time I came round to having to do an American accent it didn't feel like that much of a stretch.'