The Irish Nurses and Midwives Organisation (INMO) says the downgrading of the Children's Hospital at Tallaght has come out of the blue and will severely impact on hospitals elsewhere.
The facility is back open today after services were relocated in March because of Covid-19.
However there won't be a Paediatric Emergency Department, with critically ill children redirected to Crumlin and Temple Street hospitals.
This week, Crumlin reported its highest number of patients on trolleys since the pandemic began, as 13 children went without beds on Wednesday.
INMO Industrial Relations Officer for both CHI at Crumlin and CHI at Temple Street Hospitals, Mary Rose Carroll said members have been caught unaware with the change.
"They have not been given extra resources but are expected to cope with thousands of extra acute patients.
“Yesterday in Crumlin, the emergency department was overwhelmed. Thirteen children had to go without beds after being admitted.
"Extra resources should have been in place before this change and are desperately needed now.
INMO Industrial Relations Officer for CHI Tallaght, Joe Hoolan, said staff and clinicians have been left scrambling to develop safe pathways of care for services in Tallaght, Crumlin and Temple Street.
"“This is a sudden and unplanned change. Reconfiguring services in such a frantic manner is simply unsafe," said Mr Hoolan.
"Despite the hospital’s statement implying that this was planned, it has come as a major shock to the frontline.
“INMO members only became aware of this when they arrived to work on Monday morning, with no prior warning or discussion.
"We’ve seen these kinds of no-warning changes made previously in the Northeast and Midwest, but clearly no lessons have been learned."