YOU don't have to speak long with Mary McAleese – a nanosecond, maybe two – before realising that her Christian faith, above all else, is what drives her.
Burning, alive and bright, she absorbs it like sunlight, photosynthesising it into the energy which has fuelled a life that has already seen her become a significant public figure in law, academia and, most prominently, as president of Ireland for a bridge-building 14 years.
Anyone would have been happy to have achieved a portion of what Mrs McAleese has in just one of those fields; yet there is a sense in which it has all been preparation for the vocation she is now pursuing, that of making a serious contribution to the debate around the future shape of the Catholic Church.
In doing so, she has become synonymous with the sort of causes which, depending where you stand, make her either a progressive hero or a liberal who makes other devout Catholics blanche. Faithful, certainly, but unafraid to rebel where she believes the Church has got it wrong.
She is keen to see the priesthood open to women, for example, and played an influential role – some have argued that as a former president, she was too influential – in the referendum debate which earlier this year redefined marriage: "An impressive step by the Irish people to put true equality into our constitution," she writes in her foreword to the recently published book Ireland Says Yes: The Inside Story of How the Vote for Marriage Equality was Won.
As president she did the same, challenging cherished shibboleths and leading in new directions, such as when she controversially received communion in a Church of Ireland service and set about forging a mature, peaceable relationship with Britain and Queen Elizabeth II.
On departing the Aras in 2011 she immersed herself in canon law, studying in Rome at the Gregorian University.
This is not what former heads of state normally do. She could have pursued – presumably lucrative – opportunities elsewhere and in any case, as PG Wodehouse's Sir Roderick Glossop observed, "a lay interest in matters to do with liturgical procedure is invariably a prelude to insanity".
Mrs McAleese is saner than most. In double-quick time she published Quo Vadis?, a dense little book lobbed like a brick through the stained glass window of the Church's moribund centralising instincts.
The Second Vatican Council had envisaged a collegial Church in which the Pope and bishops would reach decisions together and – crucically – in consultation with the laity.
This renewal of Church governance ran into the sand under Pope John Paul II and Benedict XVI, with no structures to capture the mood of the people ever established: is it any wonder, argues Mrs McAleese, that the Catholic Church can seem remote, distant, monolithic?
She detects the possibility of change under Pope Francis, not least because he actively encourages debate: "The Catholic intellectual world has been suffocated by this inability to talk, debate and discuss, to push the envelope without being seen as being heretical or schismatic," she told the Irish Catholic this month.
Those Vatican II years were hugely influential on the young Mary Lenaghan as she grew up on the cusp of the Troubles in Ardoyne in north Belfast.
Many of her friends were Protestants, and thinking about the landmark meeting in 1966 between the then-Archbishop of Canterbury Michael Ramsey and Paul VI, at which the Pope gave the Archbishop his episcopal ring, still sends a shiver down the spine.
The Passionists at Holy Cross were central to nurturing her faith. One priest in particular made an enduring impact: "Apart from Christ, Fr Justin Coyne has been the biggest influence on me."
He died aged 49 on June 22 1969 – "it was the week the Troubles broke out and a few days before my 18th birthday" – and is remembered as being "utterly beloved by the children of Ardoyne".
"He had the most extraordinary and wonderful way with children," says Mrs McAleese.
With the clerical abuse scandal in mind, the worst revelations of which shadowed her presidency, she adds: "Saying that now sounds awful but he was the perfect antidote to the garbage we have lived through."
Fr Coyne visited her home every Sunday after evening Mass. "He would come in and ask us children what did we think of God, what was our view of heaven, what did we think of these things.
"He never ever told us, 'Here's what you must think.' He told us, 'Let's talk about what you think.'
"And then gradually, building on what we thought, he would very gradually teach us."
From Fr Coyne, "I learned so much about the God that I believe in and learned to love the Church that I love".
She met her future husband Martin four days after his death – "I often think Justin had a hand in that" – and she promised "the day he died that if I ever had a son he would be Justin – and he is".
Justin is gay and while that undoubtedly influenced her support for the Yes vote in the marriage referendum, her involvement in this area goes back 40 years to the campaign for homosexual law reform.
It is her faith that informs her views on sexuality and equality, as it does on every other subject: "What infuses me, what is the essence of my being, is my faith in Christ."
At the start of next year she takes up a six month post as Distinguished Professor at St Mary's University in Twickenham; later, she will return to Rome to complete her canon law doctorate.
As she leaves Ireland once again, thoughts of St Columbanus, one of her heroes of the faith, will likely cross her mind.
He left Bangor 1,400 years ago on a mission to spread Christianity across Europe and Mrs McAleese has made a documentary about his story and legacy.
It is an appropriate fit: like Columbanus, Mrs McAleese's vision is broader and more far reaching than these humble shores. Her influence will also be felt for years to come.
:: Mary McAleese & The Man Who Saved Europe, an RTE/BBC documentary about St Columbanus presented by Mrs McAleese and produced by Declan McGrath, is on BBC One on Monday at 9pm.
