However, the Killiney native, who began playing the part of cheeky chauffeur Tom Branson in the fourth episode of the first series, was initially reluctant to speak in his own accent -- as he didn't want his character to be stereotyped.
"When I went for the part, speaking in a northern English accent, the character wasn't Irish at all. They changed it for me. When I got it, they said they would keep my Irish accent for it, but I was wary. I didn't want to be another typecast as Irish Free Stater, it's been done before," he said.
"I told producers that I was very aware of us not falling into stereotypes and they said no, we're not going to go down that route.
"That in a way made it all the more interesting. While he is Irish, he's not plucking the same strings that we've seen countless times before on TV."
Leech, who also starred in Man About Dog and once beat Colin Farrell to the title of Sexiest Irish Male, also said that he's relishing playing the part of Branson, who's responsible for leading blueblood Lady Sybil astray.
"I'm really enjoying it. The set and the costumes look just brilliant," he said. "It was a little daunting because of the names attached. You arrive on set and then day two you're driving Dame Maggie Smith around! This is honestly one of the nicest casts I've ever worked with."
Set in 1912 in an Edwardian country house, Downton Abbey is the home of the Crawley family and delves into the intrigues and dramas of both the gentry and their servants.
The show is winning huge ratings.