The 21-year-old leaves Celtic after spending the last five years in Glasgow.
Oxford United have announced the signing of Fiacre Kelleher from Celtic.
The Irish centre-half has joined the League One club on a two-year deal.
Kelleher had been on Celtic's books for the last five years but it is clear that he is not in manager Brendan Rodgers's long-term plans.
The Hoops have Kolo Toure, Jozo Simunovic and Erik Sviatchenko as centre-back options and the Northern Irishman obviously felt that the Cork-born man was not ready to compete with that trio for a starting berth next season.
Kelleher spent last season on loan at Peterhead and managed three goals in 25 appearances for the Scottish side.
He had played regularly for Celtic's Under-20 side, but at his age senior football is important to aid is ongoing development.
He told Oxford's official website after completing the move: "I’m really pleased to be joining. It’s a fantastic club and they have done really well over the last few years.
"At this stage in my career I want to test myself, be part of the first team and play regularly. Now I’m looking forward to joining up with the lads in pre-season.”