Fianna Fáil health spokesperson Wicklow T.D. Stephen Donnelly says staff shortages in hospitals mean nurses have been left with little option but to strike.
Nurses and midwives are mount the largest strike in the history of the health service this morning.
37,000 INMO members are on the picket lines outside hospitals and HSE facilities - demanding pay increases and government action on staff shortages.
The government says wage hikes for nurses would open the floodgates for other unions to seek pay claims, which the State cannot afford.
25,000 medical appointments have been cancelled and the HSE is appealing to people not to attend emergency departments unless absolutely necessary.