Up to 20,000 pilgrims have continued a 1,500-year-old tradition by climbing Croagh Patrick in Co Mayo.
Some 16 people were seriously injured in falls on the mountain as climbers had to deal with cold and damp conditions.
Hundreds of pilgrims began the climb in the pre-dawn hours making their way to the mist-clouded mountain in the dark.
The first in a series of masses was celebrated on the summit at 8am.
The Archbishop of Tuam, Dr Michael Neary, spoke to pilgrims of the importance of maintaining the faith that had kept the tradition alive through many difficult centuries.
He said that faith was never more important than at a time of much doubt and uncertainty and concern for the future.
Climbing conditions on the mountain were difficult for most of the morning with thick mist and low cloud on the summit.
The Aer Corps was unable to provide assistance because of mist.
A mountain rescue spokesman said that the casualties had occurred mainly on a slippery stretch between two bends about three quarters of the way up.
