Some 47% of Irish define themselves as Catholic, against 69% in 2005, according to the results of a recent survey.
Ireland is also one of the 10 countries with the highest concentration of atheists.
Archbishop of Dublin and Primate of Ireland Diarmuid Martin said the drop should spur the Irish Church to "take up the challenge of renewal" and begin a new process of evangelization by participating in the Year of Faith announced by Pope Benedict XVI to coincide with the 50th anniversary of the Second Vatican Council starting this autumn.
The Catholic Church in Ireland is trying to make a new start in the wake of child sex-abuse scandals involving priests, which led the Pope to order inspections in all dioceses and religious institutes.