The Psychiatric Nurses Association is to escalate its industrial action with a three-day strike on 19, 20 and 21 February.
Up to now the PNA has only engaged in an overtime ban, though that has caused disruption in mental health services.
However, its 6,000 members will strike for three consecutive days next week in tandem with the 37,000 members of the Irish Nurses and Midwives Organisation.
The INMO is already scheduled to strike on 19 and 21 February, but the PNA move is likely to create serious challenges in mental health services.
The move comes as the HSE has voiced concern about the ability to guarantee safe care to patients amid mounting disruption caused by the industrial action.
One source indicated that the situation had moved from concerns about inconvenience to concerns about risk.
General Secretary Peter Hughes said the decision to escalate reflected the deep frustration of members at the lack of resolve and urgency by the Government to bring forward realistic proposals that acknowledge the extent of the recruitment and retention crisis in nursing and which will deal with the issue "once and for all".
Asked why the PNA is not attending the Labour Court talks in the nurses' dispute, a spokesperson said the association had not been invited, and was "not happy about that".
The PNA is not affiliated to the Irish Congress of Trade Unions.
The INMO and the PNA are holding a rally and march in Dublin tomorrow to highlight their grievances over pay and staff shortages.