The number of nursing positions vacant in hospitals and other healthcare settings rose slightly to 1,154 in the final quarter of last year, latest figures show.
This means 23 more posts were vacant at the end of December than at the end of the previous quarter in September.
The vacancies have to be covered by agency nurses, which it claimed cost more, and by paying staff nurses to work overtime.
A total of 980 nurses worked overtime in the last quarter of last year and 812 agency nurses were employed to cover the vacancies over the same period.
The figures, which were collected during January and February this year, have now been published by the Health Service Executive Employers Agency.
Its latest national survey of nursing resources shows a significant level of movement of nurses within the system.
"Employers reported that a total of 970 nurses were recruited in the quarterly period October to December 2005 and that a total of 756 nurses resigned/retired/ moved to another employer in the same quarterly period," it said.
Some 465 of the nurses recruited during the period were recruited from abroad.
The report indicates the largest number of vacancies were in the eastern region. And the highest number of vacancies in an individual hospital was at St James's Hospital, Dublin, where 55 posts were reported to be vacant at the end of December last.
Meanwhile, the report says the total nursing workforce also increased over the period under review to 35,248. The number of positions vacant represent 3.27 per cent of this total.
The president of the Irish Nurses Organisation, Madeline Spiers, said she believed many nurses were leaving the public system because they were overburdened.
"They are bleeding into the private system where they don't have A&E units and where workloads are controlled. They are also travelling abroad," she said.
The decision by the Health Service Executive to employ nurses graduating this September was, however, a welcome step, she added.
Phil Ní Sheaghdha, industrial relations officer with the INO, said it was often the case with current nurse staffing levels that one nurse would have to look after 30 patients on night duty. It was driving nurses away, she added.
Furthermore, she said, staff ceilings in the health sector meant permanent nursing posts were being left vacant and agency nurses used instead.
"It's costing taxpayers a fortune and it does not guarantee patients continuity of care," she said.