O'Shea had been working with Ireland's youth side since 2020 but has now earned a promotion.
The 41-year-old also works as a first team coach at Stoke City.
O'Shea played 118 times for the Republic of Ireland national team.
He won five Premier League titles and the 2008 Champions League with Manchester United.
Speaking about O'Shea's appointment, Ireland manager Stephen Kenny said: "We're delighted to welcome John to the senior international coaching team.
"John had a brilliant career playing for Ireland right through from U15 to earning 118 caps for his country. Following that, he has committed himself to a career in coaching, working with both Reading and Stoke City for almost four years now.
"He has also combined that with a full campaign with the Ireland U-21s team under Jim Crawford and I have watched his progress closely as he made a positive contribution throughout his time with the team. John will now make the transition into the senior coaching team where he knows the team well and we look forward to John joining our staff for the Latvia and France matches in March."
O'Shea said: "I'm delighted to join the Ireland senior international team coaching staff and looking forward to working with Stephen and the rest of the backroom team when the campaign begins in March.
"It was always an honour to play for my country and put on the green shirt so to join the coaching staff and work with the current group of players and staff is an amazing opportunity for me.
"I'd like to thank Jim Crawford and the Ireland U-21 backroom team who've been fantastic to work with over the last few years and wish them every success after such a strong qualification campaign last year.
"We've got a really exciting group of players, a good mix of youth and experience, and some fantastic fixtures to look forward to this year so I can't wait for it to begin and to get to work with the squad and staff."
