Nurses at Midlands Regional Hospital in Mullingar are to ballot for industrial action next week.
Last week, nurses at the hospital staged a lunchtime protest calling on the HSE to bring an end to the overcrowding there.
They want more staff, more beds and an improvement in working conditions. Any action would take the form of a work-to-rule.
A total of 518 people were waiting on trolleys in hospital emergency departments across the country today, according to latest figures from the Irish Nurses and Midwives Organisation.
In University Hospital Limerick alone, 54 were awaiting admission.
Fianna Fáil health spokesperson Billy Kelleher said: "Once again, we see the situation reaching critical levels, and there is now a real fear that the numbers on trolleys could match or even exceed the record-breaking 601 figure registered last month."
Earlier, the INMO it had secured more than 100 extra nursing posts for hospitals in the mid-west and Kildare.
Seventy staff will be recruited in Limerick, Ennis, Nenagh and Croom, while 39 nurses will be hired at Naas General Hospital.
The INMO says the extra posts were secured at two separate hearings on the overcrowding crisis in these hospitals.
Nurses in the mid-west have postponed industrial action, which was due to begin this morning.
The INMO is recommending that its members in Naas defer industrial action, which is set to start tomorrow.
