Diarmuid is also a senior coach for Northern Ireland under former Magpies player Michael O’Neill and will continue his work there during international breaks.
The Irish coach began his coaching career in women’s football with Crusaders and Glentoran then ventured into youth coaching positions in the United States before becoming an assistant to Stephen Robinson at Scottish sides such as Morecambe and St Mirren.
Diarmuid spoke on how excited he is to take up the role and how he never expected an opportunity like this.
"I'm really excited to come in. It's a role I'm really looking forward to getting stuck into.
"The club speaks for itself in terms of the size, stature and fanbase. It's a really exciting time now as well with the ownership and everything that is planned going forward.
"The opportunity to come in is something I would've never expected, but when it come across my desk, it was a really exciting one and a quick process."
United’s Academy director, Steve Harper spoke on the appointment and how Diarmuid will impact the club.
“I’m delighted Diarmuid has accepted the opportunity to become Lead Coach of Newcastle United Under-21s.
“He is a young, progressive and ambitious coach, and will play a key role in supporting our talent pathway into the first team.
“I’d also like to thank Jack Ross, Neil Winskill and everyone connected with the Under-21s for their hard work and collaborative approach while we recruited into this position.
“We have gone through a rigorous process, and in Diarmuid, we have an exceptional coach and person joining us. I’d like to welcome him to the Newcastle United family.”
United’s sporting director, Paul Mitchell also spoke on the Irishman’s knowledge and experience which will bring a lot of quality into the Magpies backroom staff.
“Diarmuid is a very highly regarded coach and is a great fit for the profile we have been looking to recruit.
“He has amassed a wealth of experience during the formative years of his coaching career, working across youth, men’s and women’s club football and at international level.
“The Academy is a cornerstone of our sporting strategy, and Diarmuid’s role is a crucial one in preparing players for the next significant step in their development journeys.
“The Academy has produced exceptional players over recent years, and we look forward to supporting Steve Harper, Diarmuid and our Academy team in continuing to build on that excellent record.”