The 22-year-old Lafferty was with the Clarets on trial last week and played for us in the behind closed doors game at Wolves and he impressed enough for Burnley to give him a two and a half year contract.
"He's a player we have known about for a while. We had a look at him for a week in training and he impressed us," said Clarets' boss Eddie Howe.
"It's the closed-season in Ireland so he's a bit behind in fitness work so we have a bit of work to do to get him right up to speed and it is a big jump for him from the level that he's played at, but he's had a good grounding at Celtic. You can see he's had a good education. He's got a lovely left foot and he loves to get forward.
"He's a really good footballer and he fits very much into our philosophy. He's one for the future but is also in our thoughts for now."
Lafferty has been signed from his home town club but started his career in the SPL at Celtic during which time he spent time on loan at Ayr.
He then returned home to Derry and was in the team that won the League of Ireland Cup last season whilst finishing third in the league.
As someone on the message board recalled today; the last left back we signed from Ireland was Alex Elder and he didn't do too badly.
On completing his move, Burnley's latest recruit said: "I think I know what it takes to make it over here and I just hope I can hit the ground running,
"It's a dream move for me making the step across the water and being given this opportunity. It's one I want to make the most of. It means a lot to me. Burnley is a massive club, being in the Premier League just a couple of years ago, so I just hope I can hit the ground running and try and help the team's cause for the play-offs this year.
"I was here for a few days and was very impressed with the set-up and I'm really looking forward to the experience here."