The 36-year-old got up close and personal with the pooch on the US film set.
The Castleknock native will call on his Irish roots for his latest movie, as he plays the role of Irish burglar Peter Lake in the film adaptation of Mark Helprin's 1983 novel.
The film follows the story of Peter, who falls in love with a dying woman, played by Downton Abbey's Jessica Brown Findlay, when he breaks into her mansion in Manhattan.
The film has seen Colin having to grow his hair long for the 19th century story.
A grandfather shirt and tailored jacket were also part of his wardrobe.
Will Smith and Russell Crowe join Farrell in an all-star cast for the fantasy story, which is set in 19th century Manhattan.
American actress Jennifer Connelly, who won an Oscar for her role in the 2001 hit Beautiful Mind, will reunite with her co-star Crowe for Winter's Tale.
The film will include scenes of Colin riding bareback on a white pony.
The actor learned to ride a horse a few years ago for the blockbuster Alexander, which also starred Angelina Jolie and Val Kilmer.
The Dubliner has a busy workload at the moment with several films in the pipeline, including Saving Mr Banks -- a behind-the-scenes look at the making of the Disney classic Mary Poppins.