On leaving Celtic, O’Riley said: “It is hard. People probably don’t realise how big the club is until you get up there. It’s a real family club and one that I was made to feel welcome at very early on.

“It’s a very unique place just in terms of the passion the fans had, [it] is something I had never experienced before. I thoroughly enjoyed my time there.

“It is still a little bit emotional speaking about it because I had such good memories there, but it just felt like the right time for a new challenge for me.”

Brighton manager Fabian Hurzeler said O’Riley’s abilities as a box-to-box midfielder made him “special”.

“He is a great character, and has a good ambition to succeed and that’s something which is very important for us,” Hurzeler added.

O’Riley becomes Brighton’s seventh signing of a busy summer that includes a £40m club-record deal for French forward Georginio Rutter from Leeds and Netherlands midfielder Mats Wieffer joining from Feyenoord for about £25.5m.

Hurzeler’s side are second in the Premier League after beating Everton and Manchester United in their opening two games of the season.