Almost 5,800 patients in the west and northwest had their operations, procedures or appointments cancelled in the first three months of this year.
It relates to the Saolta University Health Care Group, which includes Roscommon, Portiuncula, Sligo and Galway university hospitals.
It emerged yesterday Galway and Sligo hospitals were among the five most overcrowded in the country last month.
Elphin councillor Valerie Byrne, from the Roscommon Hospital Action Committee, says many people with serious illnesses are affected by cancellations.
She said: "You have people that, if they got their cancer treatment, they could be saved. That's where it's at.
"This needs to be dealt with immediately. Five thousand in the first three months - I am really, really shocked at the size of that.
"I am sure some of this comes back to (recruitment) embargoes. They are lifted in some places but not in some places.
"We are so short of doctors, nurses and consultants."
