Latest: The INMO Executive Council is meeting tomorrow to discuss further industrial action on top of the five days already announced.
37,000 nurses staged their first 24-hour strike on Wednesday leading to thousands of appointments being cancelled.
The union says the government has not made any "serious proposals" to avert more strikes and say they have been forced into this position.
Nurses and midwives do not want to be on strike for one minute, let alone several days.
But sadly we have been forced into this position by a government which refuses to listen.
We are immensely grateful for all the public messages of support. Thank you! (2/2)— Irish Nurses & Midwives Organisation (@INMO_IRL) February 1, 2019
Meanwhile, 6,000 members of the Psychiatric Nurses Association (PNA) staging an overtime ban.
This is the second day of action and the union is set to hold further action next week and three days of strike action from February 12.
The PNA has today said that the gap between Psychiatric Nurses and the HSE "remains as wide as ever".
General Secretary of the PNA Peter Hughes says talks with HSE have failed to produce results.
"We had engagement over three days last week under the auspices of the Workplace Relations Commission and there was no realistic proposal that came from the employers," said Mr Hughes.
Earlier: Psychiatric nurses hold a second day of industrial action today as part of their dispute with the HSE over pay and staffing.
Thousands of members of the Psychiatric Nurses Association (PNA) are staging an overtime ban.
The union says nurses will refuse to do additional hours at a significant number of locations nationwide.
They are due to join nurses and midwives in three days of strike action from February 12.
"The making of this is not ours, the HSE have not recruited and retained sufficient nurses to provide a safe, comprehensive service for patients," said Peter Hughes, General Secretary of the PNA.
The PNA says its members have been left with no option but to engage in industrial action.
While the Public Service Pay Commission has said it does not believe that pay has anything to do with the retention of staff, Mr Hughes disputed this, adding that vacancy rates have increased by 40% over the past year.
He said that any proposal would be brought to members for consideration, but as yet they have not received any despite spending three days at the Workplace Relations Commission last week without any negotiations taking place.
Mr Hughes said that overtime is voluntary and therefore cannot be construed as industrial action.
The issue can be resolved and there should be no penalty on staff who take part in the ban.