The first mass of Reek Sunday got underway around 8am this morning.
The Archbishop of Tuam, Michael Neary is leading the annual Croagh Patrick pilgrimage which has been a feature of the Irish Catholic Calendar for the last 1,500 years.
More than 15,000 pilgrims - some with bare feet - will today climb to the summit of the County Mayo Mountain, where mass will be said every hour until 2pm.
Jimmy Barry from South East Mountain Rescue said it is a daunting challenge.
He said: "When you're heading up here today there is going to be showers of rain so the rocks are going to be slippery, it's going to be a hard climb.
"I hope and pray that today is going to go well for everybody and that we are not busy at all, but that is usually not the case."
Stay Hydrated and Go at Your Own Pace
Order of Malta Ambulance Corps urges pilgrims to prepare for warm weather and high volume of people ahead of Reek Sunday climb at Croagh Patrick this weekend. #SavingLives #ReekSundayhttps://t.co/VUet5ICAlA pic.twitter.com/Hqq8g2CoV5— Order of Malta IE (@OrderofMaltaIRL) July 26, 2018
The recent fine weather is expected to boost numbers, however, Keelan Moran from the Order of Malta Ambulance Corps says pilgrims should be prepared for all seasons on the mountain.
He said: "It is a very tricky event to dress for.
"We would advise everyone to be prepared for every eventuality, particularly with the hot weather at the bottom.
"Often times towards the top of the mountain it can be over 10 degrees lower in temperature and it gets very rainy, misty and windy at the top.
"While you can have your t-shirt on you at the bottom you're going to need warm jackets and rain gear at the top."