Almost 7,000 patients were being cared for on trolleys in hospitals last month.
The Irish Nurses and Midwives Organisation (INMO) warned of "a deepening crisis" in which the numbers of people on trolleys in hospitals in October was 34pc higher than in the same month last year.
The union's Trolley-Ward Watch survey revealed that 6,977 people who were admitted to hospital last month found themselves on trolleys, either in overcrowded emergency departments or in wards.
The biggest number of patients on trolleys - 658 - was at Beaumont Hospital in Dublin. Our Lady of Lourdes in Drogheda had 631 and Connolly Hospital in Blanchardstown had 570.
"For the fourth month in a row the level of overcrowding in emergency departments across the country has continued to increase when compared to last year," said INMO general secretary Liam Doran.
"There is no doubt that this worsening situation arises as a direct result of the wholly-inadequate budget given to the health service in 2014 for ill and vulnerable people.
"There is no excuse, when this economy recovers and the Government speaks of growth and economic green shoots, to have growing numbers of ill people on trolleys with no privacy and dignity.
"In order to begin addressing this critical situation, the INMO is now calling on the Government and the HSE to immediately bring forward the €25m which is allocated to address delayed discharges in the 2015 Budget so that it is available immediately to provide additional beds, home care packages and frontline staff.
"We are now entering the peak winter period. These hospitals, with severe and unacceptable levels of overcrowding, will simply not be able to cope with the inevitable increase in demand which will occur.
"This is a critical situation and it requires immediate action."
Meanwhile, nurses at Our Lady of Lourdes will hold a lunchtime protest on November 19 to highlight severe overcrowding there.