The former Shelbourne, Bohemians and Hibernian boss has replaced Trevor Croly as manager at Tallaght with immediate effect
Fenlon, who has enjoyed spells in the League of Ireland with Shelbourne, Derry City and Bohemians, as well as a stint in Scotland with Hibernian, takes of the reins at the Tallaght club following the departure of Trevor Croly.
"I am delighted to be taking over as manager of Shamrock Rovers and am very much looking forward to getting to work at the club," Fenlon told the club’s official website.
"The team is in a good position, and so we’ll be looking to push on and build on that. I am familiar with the team having seen them numerous times this season and so we will get to work straight away.
"I’m looking forward to the game against Derry City on Friday night. Hopefully there will be a big crowd and we will work hard to get a result from the game and start on a positive note."
Shamrock Rovers chairman, Jonathan Roche welcomed Fenlon to the club and, emphasising his experience, said that he is "the right person" to take over as the Hoops strive for European qualification.
"There are just three points between us and third place going into the last round of fixtures, and we have a big game coming up against Derry City on Friday night. So it was very important that we moved quickly to put a manager in place to make sure that he has the chance to hit the ground running," Roche said.
"European qualification is extremely important to the club and so we need to keep the pressure on in that regard.
“Pat is the right person for the job at this time and so there was no need for delay. Pat has considerable experience at this level and comes into a club that is building good football structures and has a good squad of players.
"It has been a disappointing couple of weeks for everyone involved, but we need to move on quickly. Everyone at the club needs to pull together now and battle for every point for the remainder of the season."
The 45-year-old will take charge of his first game at the club against one of his former employers, Derry City, on Friday, August 8.