It started simply enough. Brian McCarthy was a school teacher and volunteer coach in Kerry who, inspired in his own youth by the genius of players like George Best and Johan Cruyff, was anxious to get across to a team of schoolboys how he would like things done, so he decided to rig up a bit of video equipment and show them some tapes of Ajax in action.

It all seemed to go rather well with Tralee Dynamos winning their first (and last) FAI Youth Cup that year. Scarcely 12 months later he was working for Brian Kerr, then looking to build a network of progressive coaches for the FAI and keen to encourage his enthusiasm for what was then a fairly primitive form of video analysis.

By early 2003, Kerr was the Ireland manager and McCarthy was one of the first names added to his backroom staff. Since then he has been at the heart of the set-up as coaches came and went, racking up 131 senior games. The run finally has finally ended this month, though, with McCarthy, after 15 years at the association, leaving to take up a new challenge, as they say. In his case, the opportunity is a mix of the personal and professional. He and his wife will be joining their son and his family in Sydney where he will become Director of Sport at Newington College, a prestigious private school with a reputation for producing stars and the wherewithal to attract the talent required to keep doing so.

McCarthy’s public profile to date has been almost zero. Amiable and endlessly polite, he would be seen at training and games, wandering about with his cameras or at work, filming, before he quietly slipped away to specially kitted-out rooms at the team hotel to do the behind the scenes stuff.

This, he acknowledges, has changed almost beyond recognition; from hitting rewind and play over and over again on a VHS recorder in his Dynamos days to working with software costing thousands of euro. Attitudes have changed too with many players bewildered by the barrage of information in those early days but all of the better ones basically hungry for it now. Everyone, he says, has learned along the way.

“That doesn’t seem that long ago,” he says of the 1997/98 season when he first unleashed the Ajax tapes, “but in terms of this analysis, it’s like three centuries.”

‘Paralysis by analysis’

Everything was better by the time he was working with Kerr and the senior players but not, he admits, quite good enough.

“I know there was an article at the time, what was it . . . ‘paralysis by analysis’, which was maybe [how it was viewed by] some players. I still have it at home, the article, I’m sure he [Kerr] has it as well. But anything new that comes into football, whether it’s strength and conditioning, it does take its time to be accepted and we were busy trying to find a whole new way of doing things and trying it out.”

“There are no complaints now. If you tried now to prepare a team, club or international, without that analysis, they would come to you en masse, demanding to know what the hell is going on . . . ‘we haven’t seen the opposition’s set pieces,’ or whatever. It’s gone the other way now, it’s funny how things have changed but we were learning. It was a whole new technology and when that’s the case you’re probably not aware of how finite the time-span should be; you’ve got to boil it right down.”

Later, Giovanni Trapattoni had, he says, great instincts for what was required, with the Italian calling for "the smallest details," and telling McCarthy to "narrow it right down; too long, too long, too short."

What might have been a lengthy session for the entire squad at one point became a series of tailored viewings for players of different positions. Everything became more and more precise. These days, he says, 10 or 20 games may be cut to a matter of a few minutes to illustrate a handful of core points.

The purpose of it all, though, is manifold. When McCarthy took on the role there was next to nothing in terms of an archive and he had to cadge what he could from broadcasters in order to build a back-catalogue. Now every Ireland session and game is recorded for use, not just with the senior players but also with the underage teams. A specialised software package SportsCode is used for the analysis itself while a subscription website like Wyscout can provide footage of virtually any opponent from around the world.

Increasingly important

With so many possibilities, analysis has become more and more central to a professional team’s preparations. Recent games are analysed for mistakes and so training sessions are shaped to address shortcomings, then themselves analysed. Prospective opponents are assessed collectively and individually too and sessions developed to prepare players, often in minute detail, for what they will face.

After the game, against a background of broader preparation and tactical work, the cycle begins again and McCarthy speaks with obvious admiration of how managers like José Mourinho and Pep Guardiola process so much information then oversee, routinely in a very hands-on way, its application.

When things go wrong during games, though, the manager’s first task is to try to address the situation there and then and video can be a key asset for attempting to do that in the dressing room.

“We have it live in there. It’s a fantastic aid. You [the manager] might be trying to explain that but they mightn’t even agree with you. You might say to the centre half that ‘you’re not doing that’ and he might say ‘I am’. You say, ‘well have a look at that now’.

“It’s visual evidence and the players are great once they see the evidence [but] if you are going to international players or top level players from any sport with misinformation or partial information as a manager, you’re dead in the water.

“The minute you start saying to me that I [a player] did something wrong, I say ‘you’ve got to show me the evidence,’ because if I think I didn’t do it I’m going to be telling you straight to your face and if you want a row I’ll have it.”

Like the managers, though, the best players, he says, embrace it even if there is some variation in the way they take it. Damien Duff, he recalls, used to come far in advance of Euro 2012, looking for whatever he had on opponents. Richard Dunne was more inclined to look for a general refresher.

Sometimes, inevitably enough, mistakes are still made. McCarthy, who clearly likes all of the managers he has worked with, is fond of many of the players and is immensely proud of the work he has done, can think of a few instances where individuals did something they were specifically warned against and goals were conceded and believes that in those situations “the management is entitled to ask ‘what were you doing?’”. This they do, presumably in more colourful terms, but even now, he is reluctant to name names.

Other times the responsibility is more collective but it is captured by the camera nonetheless, as for the opening goal in the 6-1 against Germany. McCarthy recalls: “when we did the analysis and looked at it in slow motion the manager went absolutely berserk with them; Seamus [Coleman] was the right back with the ball and then we see that the two sitting midfielders are here [he signals somewhere advanced], and we see that the left back has also gone up when he should be back here . . . as a manager you just lose your life.

Much more recently, Ireland played Germany again and things went rather better. McCarthy ran into one of the host federation’s fabled performance analysis team.

“It’s funny,” he recalls with a smile, “the German guy was nice and we had a good conversation and at the end [just after John O’Shea equalised] he said, ‘maybe the technology’s not so important’”.