Cardinal Sean Brady chose to deliver a strong reminder of the church's position on abortion, urging those who believe in the "fundamental" right to life to lobby TDs.
He urged people to tell politicians that no government was entitled to remove that right from an innocent person.
The Archbishop of Armagh said the country was now approaching what would "prove to be a defining moment regarding Ireland's attitude to respect and care for human life".
He continued: "Public representatives will be asked to decide whether a caring and compassionate society is defined by providing the best possible care and protection to a woman struggling to cope with an unwanted pregnancy or by the deliberate destruction of another human life. I hope that everyone who believes that the right to life is fundamental will make their voice heard in a reasonable, but forthright, way to their representatives."
Cardinal Brady said politicians needed to be reminded "that the right to life is conferred on human beings, not by the powerful ones of this world, but by the Creator".
The debate around abortion has intensified since the death of Indian-born Savita Halappanavar, who suffered septicaemia after a prolonged miscarriage in late October.
In the New Year, the Government will introduce a combination of legislation and regulation to legalise medical termination as a last resort to save a woman's life.
The legislation is expected to be drafted in accordance with the 20-year-old Supreme Court ruling on the X case, which allows for abortion to be permitted when a woman's life is in danger. This includes the threat of suicide.
But the Government's position has already opened up a gaping rift with the hierarchy of the Catholic Church.
Immediately after the decision was announced, Catholic bishops accused the Government of opening the floodgates to the "intentional killing of the unborn".
Separately, both Taoiseach Enda Kenny and Archbishop of Dublin Diarmuid Martin said they had received chilling personal threats as the abortion debate raged.
Addressing Christmas Day Mass-goers at the Pro Cathedral, Dr Martin steered clear of the controversial topic to urge people to rediscover a sense of "Christian neighbourliness" and build stronger relationships with elderly and vulnerable people living close by.
Meanwhile, the Church of Ireland's Archbishop of Dublin, Dr Michael Jackson, said today's Ireland was unappealing for many – and showed "few signs of joyfulness" as the "scalpel of austerity" digs deeper.
Dr Jackson said there was also the prospect of a "wide range of embattled positions on abortion" ahead.
He said: "In Northern Ireland, there is constantly need for communities not to drift apart internally and in their relationships with others. This danger is becoming more acute in parts of the Republic likewise."
Resilience
The incoming Bishop of Cloyne, Canon William Crean, said the resilience of the Irish people would get them through the brutal economic downturn.
Canon Crean, who will take up his new post in January, was speaking in his Christmas address to his new diocese.
"Any upheaval or disturbance that this move brings to my life pales by comparison with adjustments and challenges that so many families have been experiencing this past year, and indeed years, with the prospect of further demands in the coming year," he said.
"The tragic deaths of so many people, young and old, point to a bleak landscape in social and personal terms."
He added: "The return of the curse of forced emigration is deeply disheartening for so many families and yet despite these difficulties we still journey in hope."
In his homily, the bishop of Ardagh and Clonmacnois, Dr Colm O'Reilly, said the people of Longford knew what it was like to be "enveloped by darkness" after St Mel's Cathedral was destroyed in a Christmas Day fire three years ago.
However, he added that they were determinedly working to restore the cathedral.