A safe and high quality of care for mothers and newborns would require an extra 550 midwives in the health service, the Irish Nurses and Midwives Organisation (INMO) has said.
No maternity hospital or unit in the country was meeting internationally-recognised staffing levels, it added.
The union said the internationally recognised midwife-to-birth ratio - known as birthrate plus - was 1:29.5 births.
A survey carried out in February showed this ratio was not being achieved in any Irish maternity hospital or unit, it maintained.
The INMO said the midwife-to-birth ratio in Irish hospitals and units ranged from 1:30 to 1:55. It said the national average was 1:38.
The union said the maternity unit in Portlaoise - which was the subject of a highly critical Department of Health report last week - had the poorest staffing levels in the country. The staffing-to-birth ratio there was one midwife to 55 births, it stated. The Portlaoise unit needed 33 additional midwives immediately to reach the recommended level.
The Coombe Women and Infants University Hospital, which on foot if the report last week is now to provide support for Portlaoise, was itself "severely understaffed" with a midwife-to-birth ratio of 1:40, it said.
