There are 541 patients waiting on beds in hospitals, according to today's INMO Trolley Watch figures.
The figure is up from 456 patients waiting on beds on Friday.
The figures show 378 are waiting in the emergency department, while 163 are on wards elsewhere in the hospitals.
The worst-hit hospitals are:
The action by members of the INMO follows the breakdown of talks at the WRC on Friday.
The HSE is asking the public to stay away from emergency services on Wednesday unless absolutely vital.