Andy Farrell will become Ireland's defence coach following the 2016 Six Nations after leaving his post as England assistant.
Farrell worked as former England coach Stuart Lancaster's assistant but he was not retained by new coach Eddie Jones.
The 40-year-old will now join Joe Schmidt's Ireland set-up, having worked as defence coach for the British and Irish Lions side under Warren Gatland in 2013.
"To have this opportunity to work with a very talented management and playing group really excites me," Farrell is quoted as saying by the BBC.
"With a wealth of top-class experienced senior players and a fantastic crop of youngsters pushing hard, the future is very positive for Irish rugby, and I can't wait to get started."
Farrell's contract with Ireland will begin ahead of their tour of South Africa later this year and will run until the end of the World Cup in 2019.