For once you have tasted flight you will walk the earth with your eyes turned skywards, for there you have been and there you will long to return – Leonardo da Vinci
The Heineken Cup is what motivates the elite players in Europe, and Connacht won't win the Heineken Cup any time soon.
At 31 years of age, this is likely to be McCarthy's last chance to move to a club capable of winning the competition. As a Leinster player, McCarthy will be at the forefront of European club rugby. He is guaranteed to be competing against the best players and teams in Europe on a regular basis. As a result, he will be the focus of Irish coaching staff's attentions.
For a player who only made his international debut last year – he has six Ireland caps – McCarthy is making up for lost time on both the international and club fronts.
His competition for Ireland recognition is daunting. Munster stalwarts Donnacha Ryan, Donncha O'Callaghan and Paul O'Connell (when he returns to action) are all international second-rows. Behind them in the pecking order, Ian Nagle has long been regarded as a second-row with outstanding potential.
That aspect of Leo Cullen's career is probably over, but Devin Toner is beginning to fulfil his promise with Leinster. Much is also expected from Damian Browne.
Johann Muller's influence in Ulster means that their Irish-qualified second-rows are not as prominent, but it will be remembered that, even though he has only five senior Ireland caps, Dan Tuohy did score a try against the All Blacks in 2010. He is only 27.
Iain Henderson has come to prominence as a flanker, but at 6' 6", his long-term international future could be in the second-row.
The reality of the Irish rugby scene is that those players who are regularly featuring in the latter stages of the Heineken Cup garner the headlines.
Leinster will be one of the favourites to win the Heineken Cup next season. And they will also be one of the favourites to win the 2015 competition. Connacht will not be. And that is understood to be the reason behind McCarthy's move.
He has spent the last six seasons with Connacht and his chances of winning silverware at this stage of his career diminish with every passing season.
It is impossible to be critical of McCarthy for having ambitions. That, of course, will be cold comfort to Connacht.
His status as a Leinster player – if he is a first-choice that is – will also boost his commercial earning power, but by degrees only.
He will never be able to command the earning power of the marquee names playing with Leinster. The likes of Jonny Sexton, Brian O'Driscoll, Gordon D'Arcy and Rob Kearney will always be the poster boys.
McCarthy will also be playing on a bigger stage with a more successful team and training in state-of-the-art facilities with a squad of international players. He is the only current Ireland international player with Connacht.
That said, Fionn Carr and Jamie Hagan thought the same when they moved to the Blues.
Carr, who returned to Leinster in 2011, has yet to start a Heineken Cup game in his career (he came off the bench twice in last year's campaign), while Hagan, who rejoined Leinster at the same time, has made one Heineken Cup appearance.
These players were, it must be remembered, returning to their home province, but they must have been expecting more first-team appearances when they opted to leave Connacht.
McCarthy will not want to waste his time sitting on the bench as a replacement.