One of the Ireland side that won the UEFA U-19 European Championship in 1998, Keane's only previous appearance at a senior international tournament came at the 2002 World Cup.
With three goals in four appearances there, the then-23-year-old appeared to have a glittering future.
Instead, his club career has stuttered ever since, marked by big-money moves to big-name clubs, but agonisingly short on success.
He has found it equally tough in the international side. Despite coming close on several occasions - most notably when they lost a play-off against France in controversial circumstances for a place at the 2010 World Cup - Ireland have failed to qualify for a major tournament since the 2002 World Cup and this is their first Euro championship since 1988.
Keane made an early breakthrough at Wolverhampton Wanderers, going on to represent Coventry City before a high-profile move to Inter Milan, which quickly turned sour when coach Marcello Lippi was sacked.
Keane returned to England and Leeds United, but it was not until he moved to Tottenham Hotspur in 2002 that he settled down during a six-year stint at White Hart Lane where he quickly became, and remained, the crowd's favourite.
A move to Liverpool turned into a nightmare as Keane struggled with his form and was soon back at Spurs, although this time around he failed to make quite the same impact. Loans to Celtic and West Ham United followed before he finally left Spurs after nine years and joined David Beckham at LA Galaxy in 2011.
Although he might have lost a little of the electric pace he once had, he remains a fixture in the Irish side and is the most prolific international goalscorer in the British Isles with 53 goals. Yet, he is not without his critics.
A predatory penalty-box finisher, Keane has been accused of needing too many chances to score and a tendency to hurry his shots when a calm head is needed, especially in one-on-one situations with only the keeper to beat.
It has also been mentioned that he tends to prosper only against weaker nations and that his goals are not decisive in determining the outcome of games.
But his defenders point to the seven goals that helped Ireland qualify - only Klaas-Jan Huntelaar and Miroslav Klose scored more - and his performances at the 2002 World Cup.
It was his injury-time equaliser against Germany - one of only three goals they conceded in the whole tournament - that proved key to qualification for the knockout stage.
He then confidently smashed home a last-minute penalty in the last 16 match against Spain to force extra time - only for Ireland to be beaten on penalties.
Ireland will be up against some tough opposition including Spain again, but that may suit Keane. Free of expectations, he may reawaken the raw, goalscoring player who impressed in Japan and South Korea a decade ago.