The situation in Cavan General Hospital's emergency department is being described as "perilous".
The Irish Nurses Organisation (INO) claims that up to 50 people spent the night on chairs, trolleys and beds waiting for admission.
There were 18 people on trolleys, more than twice the amount the A&E unit is designed for.
Earlier this week, Our Lady of Lourdes Hospital in Drogheda went on "protected call" due to overcrowding in its emergency department.