There were 643 patients on trolleys, or on wards - including 15 at three children's hospitals - waiting for admission to a hospital bed, according to the Irish Nurses and Midwives Organisation.
The worst affected was Cork University Hospital where there were 59 patients waiting, with 51 at the Midland Regional Hospital, Tullamore, 45 at Tallaght Hospital and 42 at University Hospital Limerick.
The INMO said there were eight patients waiting at the National Children's Hospital, Tallaght, five at Temple Street and two at Our Lady's Hospital, Crumlin.
Other hospitals affected include Our Lady of Lourdes Hospital, Drogheda with 39 patients waiting, Midland Regional Hospital, Mullingar, with 35 and the Mater Hospital with 25.
On 3 January, there were 677 patients on trolleys, which was the highest level of overcrowding ever recorded on the INMO count system.