The last thing Declan O'Sullivan needs to prove is anything to anyone, least of all himself. Not many footballers can touch his great wealth of success and experience, even in Kerry, including those labelled with loftier reputations.

Ever since Paidi O Se introduced O’Sullivan as Kerry’s new “piece of gold” in 2002, his potential - while occasionally stalling - rarely went unfulfilled. He won his first All-Ireland at 20, was made Kerry captain before his 21st birthday, and despite enduring a tortuous summer of 2006, ended up lifting the Sam Maguire after a standout All-Ireland display against Mayo.

A year later O’Sullivan became only the eighth man in history to captain an All-Ireland winning team in successive years (and the first since Tony Hanahoe, in 1977): then came three successive All Stars - in 2007, 2008 and 2009 - and all this before turning 30.

Now, O'Sullivan finds himself preparing for his ninth All-Ireland final, and whether or not he starts against Donegal on Sunday, he will almost certainly feature. That will mark his 70th championship appearance, during which he's scored 8-83. Yet against that backdrop O'Sullivan still finds himself with plenty to prove, and doesn't for one second deny his desire to get a starting place for Sunday.

“Sure, it’s been all about trying to make the starting 15, giving the management a problem when it comes to picking the team,” says O’Sullivan, who at 30 also admits he’s mellowed with age, partly by having a young family to take care of, back in Dromid in South Kerry. “This is what you train for, and play for. I have been lucky to come along with a group of players who have had a lot of success, so it’s circumstance, as much as anything, that gets you to be involved in nine All-Ireland finals.

“And there was a lot of talk, after all the retirements, and Colm (Cooper) getting injured, that maybe Kerry wouldn’t be in the picture for a while. So it has been fascinating watching the young lads, tearing into training, all very hungry for work. So, to be honest, this has been one of the most enjoyable years I have been with Kerry.

“But every player is greedy, too, wants to get the most out of his career. You wouldn’t survive if you weren’t the type of character that wanted to constantly win, every game. Certainly, for me, I knew the fellas coming through were good enough to win another All-Ireland, and I wanted to be a part of it. But I felt some onus as well, duty to the jersey after all the retirements, that these fellas needed some level of experience around them, and know how.”

So, while he may have mellowed, O'Sullivan has lost none of his competitive instinct. Yet as much as he wants to start on Sunday, he has no difficulty buying into the philosophy of manager Eamonn Fitzmaurice, where players earn their starting place, no matter how many stripes they've earned.

“I don’t think you’d last this long if that was an issue. You have to react in the right way, because it’s a test for every player. If you’re not being picked, and feel you should be, it’s all about how you deal with that. If you don’t deal with it in the right way you’ve no business being there. Eamonn wouldn’t tolerate it anyway.

“So no matter what age, what amount of experience, it’s very important to show the right character. But look, I’ve been in that situation before. Go back to 2006, I wasn’t starting for a lot of that year.”

That’s simplifying the matter: in 2006, O’Sullivan was booed by a small section of Kerry supporters, after being taken in the Munster final against Cork. Indeed he was struggling with certain commitments, and ended up leaving his job, to fully to commit to Kerry. Jack O’Connor rewarded him with a starting place in the final against Mayo, and O’Sullivan rewarded that by scoring 1-2, and making 27 plays - only Seamus Moynihan and Paul Galvin on the ball more than that.

Inevitably, given his age, mileage and some “wear and tear” on the knees, he’s had to make some adjustments. He sat out the league this year, mainly due to family and work commitments (with Liebherr Cranes), but Fitzmaurice was more than happy to accommodate that, knowing how important O’Sullivan could prove to their championship. And so it proved, especially in the Munster final against Cork, where he adopted the orchestral conductor role - or sweeper, as others might say: yet he always realised the importance of evolution, even in a traditionalist county like Kerry.

“Well the only thing that is frowned upon in Kerry is when you lose games. Once you are winning they don’t mind how you’re playing. You’d be foolish if you only had one philosophy on playing football, you wouldn’t last. And when you have a turnover of players you have to find a system that suits the players you have.”

Donegal, he says, present the greatest test so far of that evolution: “You saw that against the Dubs. It was the performance of the championship. That’s Donegal in a nut shell, they are constantly evolving. The hardest thing about playing Donegal is that positions mean nothing. They defend as a team. And they attack as a team. But we’ve moved on too. But we’ve played differently, nearly every day we’ve gone out, based on the opposition, and how we are playing ourselves.”

All of which suggests O’Sullivan could play some pivotal role on Sunday.