Roy Keane has returned to English football as Nottingham Forest’s assistant manager in a move that sees him resume his working relationship with Martin O’Neill from their time together with the Republic of Ireland.

Keane had been considering an offer since O’Neill was confirmed as Forest’s new manager two weeks ago and the former Manchester United player arrived at the Championship club’s training ground on Monday.

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The move has taken some time to go through, partly because Keane wanted time to consider whether he should accept another No 2 role or wait for an opportunity to re-establish his own managerial career.

Forest, though, worked hard behind the scenes to demonstrate their ambitions under the ownership of Evangelos Marinakis and an announcement confirming Keane’s appointment was made on Monday lunchtime.

Keane, who attended the team’s 3-1 win at home against Wigan Athletic on Saturday, has created a reputation as one of the more fascinating, controversial and unique characters within the sport and O’Neill made it clear during his official unveiling why he wanted him back by his side.

“I would dearly like him to join us,” O’Neill said. “Overall, he was great for me, and great for the [Ireland] players, and the genuine truth is that I don’t think that we would have qualified for the Euros [Euro 2016] without his presence. I have been unflinching in that support.”

O’Neill, tasked with ending Forest’s 20-year exile from the Premier League, has overseen two matches as Aitor Karanka’s successor since returning to the club where he played a significant part in the European Cup wins of 1979 and 1980. Keane’s return will invoke its own sense of nostalgia given his own history with Brian Clough.

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“I am happy to be back,” Keane said. “I am looking forward to it, it’s a big challenge and I’m ready for it. Obviously, I had great memories of my career here as a player; the club gave me my chance and I will always remember them for that. But it is a different challenge now.”

Signed from Cobh Ramblers for £47,000, Keane was one of the most astute pieces of business made by Clough during his 18-year reign at the City Ground, culminating in the player’s £3.75m transfer to Manchester United in 1993. Keane made 154 appearances for Forest, scoring 33 times, and was the club’s player of the year before switching to Old Trafford in what was a British record transfer at the time.