The Irish Nurses and Midwives Organisation has said there was an 8% increase in overcrowding in hospitals in July, compared to the same month last year.
Figures compiled by the INMO show that more than 5,500 patients admitted for care last month found themselves on a trolley or in an emergency department.
July is a relatively quiet month in hospitals, with fewer elective procedures taking place.
However, the INMO said five hospitals saw significant increases in emergency department overcrowding.
They are Sligo Regional Hospital, the Mater Hospital and St James's Hospital in Dublin, Our Lady of Lourdes Hospital in Drogheda, and University Hospital Limerick.
A recent HIQA report found the emergency department in Limerick was overcrowded and not fit for purpose.
Despite this finding, Limerick recorded a 75% increase in patients on trolleys in July this year, as compared to July 2013.
Sligo saw an increase of 187%, Mater 154%, St James's 154%, and Our Lady of Lourdes 99%.
The INMO acknowledges that Minister for Health Leo Varadkar cannot wave a magic wand to solve the problems.
However, it said additional funding is needed to re-open beds to deal with the demand and overcrowding.
It also said additional staff should be recruited.
INMO General Secretary Liam Doran said: "The levels of overcrowding, recorded in a number of hospitals during July, is totally unacceptable, leaves patients without dignity and privacy and causes excessive workloads on already overstretched frontline staff."
Responding to the report, HSE National Director for Communications told RTÉ's Morning Ireland: "The HSE is acutely aware of the difficulties being experienced by certain hospitals currently and is taking a number of immediate actions to deal with them.
"Central to these actions is providing an additional €5m on social care packages to release badly needed bed capacity.
"The Special Delivery Unit of the HSE has also been tasked with supporting those hospitals experiencing difficulties and where 'hot spots' exist."
Minister Varadkar said: "The vast majority of patients on trolleys now will be in a bed or at home within nine hours.
"I monitor the trolley count on a daily basis and I acknowledge that it remains a real problem in some hospitals.
"The HSE has released €5m for home care packages and long-term care to speed up the discharge of patients.
"I look forward to meeting the INMO in September to discuss ways of relieving the problem as we head into the busy winter period."