A rapid winger in his heyday, Duff will be a part of the coaching staff that is led by Stephen Kenny.
The latter is replacing Mick McCarthy in the hot seat for Ireland.
Duff is now 41 and played over 100 games for Ireland during an illustrious career.
"The calibre of Stephen Kenny's backroom team is top class and these are great additions to our senior team coaching staff," said FAI Interim CEO Gary Owens.
"Keith [Andrews] and Damien served Ireland so well as players and their experience will be such an addition to our senior squad's support team.
"Keith and Stephen have worked brilliantly together with the Under-21 squad and Damien, currently finishing his UEFA Pro Licence with the FAI's Coach Education department, has impressed everyone with his coaching skills and his switch from world class player to coach.
"Alan knows international football inside and was part of the squad at two World Cup finals. He is highly regarded as a coach with Premier League side Everton and has huge experience with Ireland.
"Together, Keith, Damien and Alan have so much to offer Stephen and his players as we look forward to the UEFA EURO 2020 Play-Offs, the Nations League and the World Cup qualifiers."