Pilgrims on the annual Croagh Patrick climb faced poor visibility and slippery conditions with the summit shrouded in mist.
Thousands took part in the age-old Reek Sunday tradition in honour of St Patrick, who it is said fasted for 40 days on the Co Mayo mountain's peak in 441AD.
Personnel from several groups including Civil Defence and Mayo Mountain Rescue were on hand to provide assistance to walkers.
Mayo Mountain Rescue said around 15-20 people suffered injuries, with at least 10 having to be taken from the mountain by stretcher.
The bulk of the injuries consisted of sprains, lower-leg and head injuries, as well as cuts and bruises.
Mayo Mountain Rescue said visibility was at 20 metres.
Peter Jordan, chairman, said: "It's very, very poor visibility. They had the Airs Corps helicopter down and they couldn't fly at all. The conditions are pretty terrible but at least it's not cold, it's quite warm up there We're not having many cases of cold or hypothermia. It's very slippy under foot."
Mr Jordan said gardai had estimated around 20,000 people took part in the walk.
Civil Defence branded the conditions "desperate", but Mr Jordan said they were not as bad as last year when walkers were battered by wind and rain.
Mass was celebrated at the summit at 8am, then every half-hour until 2pm.