The iconic five-metre steel cross at the summit of the country's highest mountain has been cut down.
The cross, which has stood on Carrauntoohil mountaintop for almost 40 years, was cut with an angle grinder, locals believe.
A group of climbers made the discovery this afternoon.
Local guide Piaras Kelly told independent.ie: "I couldn't believe it. I was hoping it was a joke and that it wasn't true and I got a shock when I saw it."
"They would have had to bring up an angle grinder or a petrol-driven consaw."
"It's an iconic structure and an iconic peak. It means so much to people of all religions, all walks of life, who have taken photos at that cross."
The vandalism has been reported to two committees who look after the Macgillycuddy Reeks, and the gardai who have launched an investigation into the incident.
The cross was erected by about 100 people in 1976 when it was assembled using a welder.
The steel cross was made by the Liebherr crane manufacturing factory near Killarney and it was carried in sections to the summit by parishioners from Beaufort.
At the time, the massive community effort was televised.
A statement from Beaufort Community Council condemned the act of vandalism. Its chairman Tim Moriarty described the cross as a landmark but vowed that it would be replaced again.
He also appealed for anyone with information to contact the gardai.
Local TD Brendan Griffin described the incident as a "shameless act of vandalism".
“The cross served as a focal point for climbers upon reaching the summit and had, in its own way, become a landmark," he said.
"Regardless of what people’s views on religion are, all would agree that this is a totally senseless act that serves no purpose except needless destruction."
Mr Griffin said he believed it was a "well-planned" act of destruction that would have required considerable preparation.
Piaras Kelly, who regularly summits the mountain, described the vandalism as "sick" and "shocking".
"The sight of the cross entices people to the summit; it's a massive structure, and when you're climbing it, you always think 'come on we're nearly there, we're nearly at the cross'."
"It's marks the high point of a beautiful summit with beautiful views. It's sick, it's shocking."
"It beggars belief how people could do this. Everybody is shocked by it," he added.