The former Waterford manager joins Jim Crawford's U-21 staff alongside John O'Shea ahead of the resumption of the UEFA U-21 European Championships qualifying campaign.
Reynolds recently left his management post at the Regional Sports Centre having guided Waterford to the Premier Division in 2017 and a fourth-placed finish in 2018.
Reynolds will fulfil his role with the U-21 national team alongside his coaching role at SSE Airtricity League Champions Dundalk.
Reynolds has an extensive playing and coaching career in the League of Ireland, and U-21 manager Crawford is delighted to bring his experience on board.
"Alan is a brilliant addition to the coaching staff and I'm delighted to have him on board as he will bring a wealth of experience to the position," said Crawford.
"Alan has achieved fantastic success during his playing and coaching career, having managed at an early age, and achieved promotion with Waterford in 2017 and a fourth-placed finish the season after.
"Also, the apprenticeship he has served as an assistant is really important having worked with Stephen (Kenny) in the past. He has a great understanding of Stephen's philosophy that we're trying to bring in across the senior and Under-21 teams.
"We're an extended staff of the senior set-up and with John O'Shea having played with Keith Andrews and Damien Duff in the past, and me and Alan having worked with Stephen, it's fantastic to build such an experienced coaching staff across the two teams."
Ireland U-21s will return to action in September with an international friendly against Slovenia on September 9 before the remaining three qualifying matches against Italy, Iceland and Luxembourg in October & November.
Reynolds, a former FAI Development Officer, is looking forward to the challenge of coaching at international level as the side look to qualify for the U-21 European Championships finals for the first time.
"I'm absolutely thrilled to come on board and really excited to join the staff. I've kept an eye on the qualifying campaign and the development of the team over the past year, and it's an exciting time for the team," said Reynolds.
"It's a really exciting group of young players which are coming through and we're looking to keep the momentum for the remaining three matches of the campaign.
"It will be my first taste of international football and there will be tough matches but I'm excited to get the chance to represent my country.
"It's important to keep my eye in, coaching day-to-day, with Dundalk and I really believe it is something which will benefit the Under-21s. It's an exciting opportunity and can't wait for the football to restart soon."