Humour and hardness. Someone who knows Martin O’Neill from the inside of a dressingroom once distilled the qualities of the next Republic of Ireland manager into those two qualities.

Tommy Cassidy and O'Neill both made their Northern Ireland debuts in 1971 and both met again many years later when coaching small, non-league clubs in England.

"I always felt he'd be a manager," Cassidy said when reflecting on O'Neill. "He had something about him. His background, his family, his education, Kilrea, Queen's University.

“He had humour and hardness, both of which you need to be a manager. Then he went to Grantham, Shepshed and Wycombe, that’s a good grounding. There’s a good mixture there. I admire him. He’s not finished yet.”

This was 2010 and O'Neill was manager of Aston Villa, about to face Manchester United in the League Cup final at Wembley. It was Villa's first Cup final for a decade. They lost 2-1, controversially, referee Phil Dowd failing to dismiss Nemanja Vidic when awarding Villa a penalty.

Villa were seen as an emerging force nonetheless, yet, five months later, days before the start of the new season, O’Neill departed, apparently after disagreements about future investment. It was a display of some hardness and it cost O’Neill more than a year out of management.

But he was not finished and in December 2011 he was appointed Steve Bruce’s successor at Sunderland.

O’Neill sparked an instant upturn in a relegation-bound team and with Sunderland being the English club O’Neill had supported as a boy growing up in Co Derry, it seemed like a fit.

But after another disagreement about the state of the squad – with another American owner, as at Villa – O’Neill had his contract terminated after just 16 months. That was at the end of this March and if O’Neill’s brief tenure at Norwich City in 1995 is considered, it can appear he does not always stick around.

In fact, the pattern is the opposite. Despite being highly-regarded and offered other jobs, O'Neill spent five years with Wycombe Wanderers, five with Leicester City, five with Celtic and he was beginning a fifth year at Villa when it ended. These are serious chunks of time in the precarious business of football management.

Add O’Neill’s playing days, most notably for Nottingham Forest with whom he won the European Cup twice, and the accumulation of time and experience mean that a young-looking 61 year-old has been in full-time British soccer for the past 42 years.

He has medals, wealth, 64 Northern Ireland caps, an OBE and widespread respect.

Born in Kilrea in 1952, one of Greta and Leo’s nine children, O’Neill comes from a GAA background. But by the time the family moved to Belfast in 1968, he was already making waves in soccer as well as in Gaelic football.

O’Neill signed for Irish League club Distillery and scored in the 1971 Irish Cup final. That led Distillery into Europe, where they were drawn against Barcelona. The teenage O’Neill, who was studying law at Queen’s University in Belfast while playing part-time for Distillery, scored against the Catalan giants too.

“I scored in the game at Windsor Park when were beaten 3-1,” O’Neill has said of that game. “We then went out to Barcelona and lost 4-0, but for us to be in the Nou Camp was fantastic. There were only 20-30,000 in the stadium, so it was sparse, but those couple of days off with the players, staying in the hotel, and that feeling that if this is what semi-professional football is like, then I want this all the time. It wasn’t long after that that I got the chance to play for Northern Ireland and go to England.”

Nottingham Forest paid £15,000 for their young Irish attacking midfielder. O’Neill stayed 10 years.

After four of those, Brian Clough was appointed manager and so began one of the most remarkable rises in the history of British and European football.

Forest had never won the league title before doing so in 1978. That would lead to two European Cups in 1979 and 1980. As well as O'Neill, John Robertson was fundamental to that team and he and O'Neill would be together as a coaching partnership up until Sunderland.

Queen’s University had to be left behind when O’Neill left for England in 1971, but Robertson dedicates a chapter to O’Neill in his autobiography: ‘The dressingroom lawyer”.

Robertson wrote: “Martin’s got a great command of the English language and the ability to deliver it in a telling fashion . . . He also has Clough’s capacity to charm the hind legs off a donkey.”

Such intelligence and charisma were required when, against a background of worsening Troubles, O'Neill was made Northern Ireland captain by Billy Bingham. Both Bingham and O'Neill deny being brave.

Under both, in 1982 Northern Ireland reached the World Cup finals in Spain, causing a sensation when eliminating the hosts.

“The camaraderie at the World Cup, considering the circumstances, was remarkable,” O’Neill has said. “Catholics and Protestants in the team, they put whatever feelings they had to the side, for the cause of the team.

“But I also think there is something about the northern Irishman, that he can take some serious situations into almost self-mockery.”

O’Neill then joked about some Protestant players singing The Sash on the team bus while he and others belted out The Soldier’s Song.

Amid the hardness was humour, which, as Tommy Cassidy said, helps explain Martin O’Neill.