After initially impressing on trial towards the end of the 2021/22 season, the 18-year-old has put pen to paper on a two-year deal.
The Dublin-born centre-back is the Academy’s second arrival of the summer, having also signed midfielder Cormac Austin.
“I’m absolutely buzzing,” Grehan said after completing his contract. “I can’t wait to get going this season, to start playing matches again, and to show the club what I can do.”
Academy Director Gary Issott added: “Seán is a talented defender who has impressed on trial and has now secured a well-deserved place at the Academy. We wish him the best of luck for his time with us.”