There are 469 patients on trolleys in hospital emergency departments, or on wards today, waiting for admission to a bed, according to figures from the INMO.
The hospitals worst affected are Beaumont with 47 waiting, Cork University Hospital with 40 and St Vincent's and University Hospital Limerick, both with 35 patients waiting.
Today nurses working in the emergency department at St Vincent's University Hospital in Dublin began a work-to-rule to highlight overcrowding problems.
The hospital has said it has contingency plans in place to minimise disruption during the action and to ensure patient safety is not compromised.
Yesterday, around 100 members of the Irish Nurses and Midwives Organisation took part in a lunchtime protest outside the hospital.
St Vincent's is one of the biggest acute facilities in the country.
The INMO claims hospital figures underestimate the true level of overcrowding.
St Vincent's Hospital has said that proposals by management to deploy additional nurses to the emergency department have been blocked by the INMO.
It says the union is insisting that the extra nurses be paid at a premium rate, which is contrary to the claim by the INMO that the dispute is not about money.
INMO Industrial Relations Officer, Philip McAnenly rejected the hospital's claims, calling them provocative and unhelpful.
