As coadjutor Archbishop of Armagh he will ease the passage of Cardinal Sean Brady into retirement.
Theoretically, Cardinal Brady will continue to lead the Church until his 75th birthday in August 2015. Few seriously believe, however, that he will still be in office by the end of 2013.
The Vatican understands that Cardinal Brady has been an impotent leader of the Church ever since revelations emerged in 2010 that he had failed to report the abuse of Fr Brendan Smyth to the police when he was made aware of it in 1975.
Truth be told, he was never the powerful and assertive leader people had come to expect in Armagh after men such as cardinals Conway, O Fiaich and Daly.
The appointment of a coadjutor is a classic Roman solution when a cardinal is mortally wounded.
Following the punishing revelations about Cardinal Desmond Connell in Mary Raftery's 'Cardinal Secrets' documentary in 2003, Diarmuid Martin was dispatched from Rome as coadjutor in Dublin.
Within months, Cardinal Connell had retired. And so the pattern will be repeated in Armagh.
Ever since the Papal Nuncio Archbishop Charles Brown arrived in Ireland a year ago the hunt has been on for a successor in Armagh.
In Msgr Martin the church has found a hard-working pastor with a keen understanding of the complex issues facing the church. While initially his role will be to shadow Cardinal Brady, the transition to power is expected to begin almost immediately.
If government officials – North or South – think that they are dealing with a cowed church, Archbishop-elect Martin has thrown down the gauntlet. In his opening remarks, delivered on the steps of St Patrick's Cathedral, Armagh, he made it clear that he won't restrict himself to the sacristy.
"There is a need for a mature relationship between church and society, in both parts of this island, and people of faith have a vital role to play. It would hugely impoverish our faith if we were expected to 'leave it at home' or 'keep it for Sundays', excluding it from our conversations and actions in daily life," he said.
His remarks will have a particular resonance given the current debate in the Republic about abortion. By attending last month's 'Vigil for Life' outside the Dail, Msgr Martin pinned his colours to the mast. It's a stance that will not have gone unnoticed in the Vatican and likely contributed to his elevation.
He will continue to speak forcefully on the issue of abortion.
But, if anyone's expecting a belt of a crozier, they'll have to look elsewhere.
"As Christians, we are not there to impose, but to invite; we are not there simply to oppose, but to convince others of the truth of Christ's teaching and to offer them the gift and message of salvation," he said.
As Primate of All Ireland he will also take over as president of the Irish Bishops' Conference.
He'll have a job of work to do uniting the bishops around him.
One of his key tasks will be to rebalance the hierarchy and reassert the primacy of Armagh over his namesake Diarmuid Martin in Dublin.
The Vatican has been fretting in recent times that the Dublin-based Martin has taken much credit at the expense of the other bishops.
They will want to see the Armagh-based Martin correct this without leaving Dublin's nose out of joint.
Ireland's bishops are due to visit Rome in the autumn to report to Pope Benedict XVI about the state of the local church.
Few in the Vatican are under any illusions about the damage done to the church in Ireland.
Eamon Martin leading a hierarchy – replete with new members recently appointed and more on the way – could be an ideal opportunity to draw a line under the toxic past.