The 41-year-old helped Stephen Bradley’s men win a fourth consecutive league crown this season – his sixth in total across two spells with the club – adding to three further major honours with the Dublin-based outfit.
Mannus started his career in the Irish League with Linfield and picked up five league titles alongside four Irish Cup crowns while also being named Ulster Footballer of the Year in 2008.
He earned a move across the water to Scottish Premiership side St Johnstone from Rovers in 2011, spending seven years at the club and helped them lift the 2013/14 Scottish Cup under fellow countryman Tommy Wright.
Now a goalkeeping coach with the Irish FA, Mannus played nine times for his country after making his debut against Trinidad and Tobago in 2004 and will hang up his gloves at the conclusion of this season by making a 286th appearance across all competitions for Shamrock Rovers.
"It’s something I know I’ll regret – no matter when I retired I knew I would regret it,” he told the club’s media channel. “It was always the intention to retire this year.
"I was going to do it last year. It went really well for me and I felt really good.
"Last year was probably one of my best seasons at the club and of my career, so I felt like I had to try and do another year so I spoke to the manager and he was happy for me to stay on.
"This year has been a bit more difficult with other things going on and I had an injury to my finger which affected things quite a lot.
"The intention was always to retire. I felt it was a good time to do it and the finger injury I’ve been struggling with quite a lot in training.
"I’ve been getting it injected to get through games which isn’t sustainable for another year.
"I was going to do it anyway but that has made it a bit easier.
"I’ll miss a lot of things but it has to happen at some stage so it’s right now.”