Grehan has penned a three-year deal with the League One club after making seven appearances for Palace's Under-21s in the EFL Trophy.
"I’ve been talking with the gaffer for the last couple of weeks and ever since I heard of the interest I was dying to come here,” Grehan said. “Seeing how well the club did last year was great and I’m just buzzing to get going.
“I think it’s the right move for me now. I’ve had a few loan spells and I just want to somewhere, settle in and play games.
“I think it’ll be great coming and playing here in League One. I’m delighted to be here.”
Big potential
Rovers manager Grant McCann said: “We are delighted to get Sean. He’s a player with a very high ceiling, he’s come off the back of really good loan at Bohemians where he excelled and he’s the current captain of the Republic of Ireland U21s.
“Sean comes to us at a great age so it was important we put him on a long-term contract so we can develop him further.
“Crystal Palace where brilliant to deal with and a huge thanks to Terry and Gavin in backing us with the transfer fee.
“Sean fits how we want to play, his character is first class, he’s excellent on the ball, defends on the front foot and is a threat in both boxes.”