Seamus Lawless, 39, was part of an eight-member expedition and was one of three Irish climbers to reach the summit, Newstalk reported.
It is reportedly believed he slipped on the balcony area of the mountain while attempting to descend to Camp 4, below the peak.
Mingma Sherpa, chairperson at Seven Summit Treks that organised the expedition, told The Himalayan Times that Lawless went missing after he reportedly fell from an altitude of 8 300 metres while descending from the summit.
"Other climbers have already descended to Camp 4 while the status of the missing Irish climber is still unknown," Sherpa reportedly said.
"Fellow climbers and base camp officials confirmed that Seamus slipped down while descending from the balcony (8 400 metres). Efforts are under way to search for the missing climber," he added.
Two team members dead
Lawless set out on the expedition with his climbing partner Jenny Shirley Copeland in a bid to raise funds for Barretstown Children's Charity, which supports seriously ill children and their families, Newstalk reported.
In a statement, Barretstown CEO Dee Ahearn said: "This is a dreadfully upsetting and uncertain time for Seamus and his family."
"Our thoughts, and indeed the thoughts of the entire Barretstown community, are with Seamus, his family and friends."
Lawless is married and has a four-year-old daughter, Irish Times reported.
Narayan Singh, a member of the maiden Indian Army expedition to Mt Makalu, died at the height of 8 200 metres when he, along with other members, was descending from the summit point on Thursday night, The Himalayan Times also reported.
The Indian Army soldier died after he suffered from high-altitude sickness on the descent, Sherpa quoted fellow climbers as saying.
Another member of the team, Ravi Thakar from India, was found dead at Camp 4 on Mount Everest early on Friday morning. "Fellow climbers found him dead inside the tent," Sherpa told The Himalayan Times.
Summits with a Purpose reported that Khumalo had made it back to Camp 4 safely after summiting the mountain, becoming the first black African woman to do so.
"Saray will rest, refuel and continue down the mountain, returning to Base Camp over the next few days - one step at a time."