20,000 to complete Reek Sunday trek
More than 20,000 people are expected to make the annual pilgrimage to Croagh Patrick.
Climbers have been warned to take care on the traditional Reek Sunday trek up the 765-metre high mountain in Co Mayo.
The pilgrimage will be led by Archbishop of Tuam, Michael Neary, and the papal nuncio to Ireland, Charles Brown.
Met Eireann has forecast heavy rain showers at times in the afternoon while top temperatures will be 17C.
The national pilgrimage is associated with St Patrick who, in 441, spent 40 days and nights fasting on the summit.
It has been carried out uninterrupted for more than 1,500 years.
Croagh Patrick has more than 100,000 visitors annually.
Some 20,000 pilgrims young and old from across the country follow in the footsteps of their ancestors, stopping at various stations on the climb to pray before celebrating Mass on the summit.
Mass will be celebrated at 8am and every half-hour until 2pm.
Climbers are urged to come prepared for the weather conditions, bring suitable warm and waterproof clothing, good footwear, a walking stick and water.