One of Ireland's most senior clergyman admitted yesterday that an imminent report on the sexual abuse of children by clergy will shock the country and reveal that thousands of children were abused by priests.

In an unprecedented homily for Holy Thursday, Diarmuid Martin, the Archbishop of Dublin, warned that the depth of the abuse "will shock us all".

The report from the commission on child sexual abuse will be published in May, and according to Martin it will throw up challenges to the Catholic church in Ireland it has never experienced before.

At a mass in Dublin's Pro-Cathedral, Martin said: "It is likely that thousands of children or young people across Ireland were abused by priests in the period under investigation and the horror of that abuse was not recognised for what it is. The report will make each of us and the entire church in Dublin a humbler church."

Organisations representing the victims of clerical sex abuse in Ireland last night welcomed the archbishop's words.

Martin is seen as a reformer sent in by the Vatican to clean up a church rocked by a decade and a half of scandals. One of the most notorious, in the mid-1990s, involved priest Brendan Smyth and indirectly led to the collapse of Albert Reynolds's government.

Accusations that the Irish attorney general's office blocked moves to extradite Smyth to Northern Ireland led to the Irish Labour party pulling out of coalition with Reynolds's Fianna Fáil and the government falling.

"We have no time to waste," Martin said yesterday. "There is a dramatic and growing rift between the church and our younger generations, and the blame does not lie principally with young people. Our young people are generous and idealistic but such generosity and idealism does not seem to find a home in the church."

He also illuminated the recruitment crisis in Irish Catholicism, in a country that once used to export its priests and nuns all over the world. "In the [Dublin] diocese there are 10 times more priests over 70 than under 40. In just a few years we will only have a little over 200 diocesan priests to minister to our almost 200 parishes."

Maeve Lewis, executive director of the One in Four organisation, which campaigns for the victims of clerical sex abuse, welcomed Martin's remarks, saying he "has been at the forefront of addressing the issue of sexual abuse within the Catholic church. He has drawn up models for child care and protection in Dublin that can be followed anywhere in the world.

"From the viewpoint of those who have experienced clerical sexual violence I think his statement and his recognition of their abuse will probably be of some comfort. For the first time, a member of the hierarchy has accepted the depth of the abuse that took place over the years".