The Mid-Western Regional Hospital in Limerick has banned visitors after an escalation in the winter vomiting bug there.
17 cases have been confirmed among patients in seven wards.
Security at the hospital has been increased and the outpatients' door has been closed to help keep visitors to a minimum.
Anyone visiting the hospital is requested to check with staff before being allowed in.
The ban does not apply to parents of sick children in the paediatric unit where the attendance of parents is part of routine nursing care.
Meanwhile, Cork University Hospital has said the visitor ban will remain in place there over the bank holiday weekend.
It is allowing just one visitor per patient daily between 3pm and 4pm.
