6,000 members of the Association of Psychiatric Nurses now have to start overtime, which is part of its growing campaign for industrial action on pay.
Psychiatric nurses initially refuse to work overtime today, tomorrow and February 5, 6 and 7.
Then on February 12, 13 and 14, they will increase their industrial action to a full strike.
The Health Service has said it doesn't know the actual impact on services today until the start of the shift this morning.
The strike by the Irish Nurses and Midwives organization against pay and staff shortages ending this morning
Today, the HSE will start canceling procedures for the next Tuesday as it and INMO are discussing emergency plans for the next 24-hour strike.
The Government has reiterated that the pay of nurses is unacceptable and violates the public service contract.
Speaking of RTÉ Morning Ireland, PNA Secretary General Peter Hughes said that mental health services are highly dependent on overtime.
He said it was not yet clear what the impact on the overtime ban would be, but he said that many community services and senior nurse management could be affected to provide coverage.
Mr Hughes said that there would be no problem with the recruitment and retention of nurses.
Although the Public Service Paying Commission has announced that it does not consider that pay has any bearing on employee retention, Mr Hughes disputes this by saying that vacancy rates have increased by 40% in recent years.
The Bring Them Home campaign, launched last year, has only received six psychiatric nurses returning from the UK.
Mr Hughes said that the PNA spent three days last week at the Employment Relations Commission without any negotiations.
He said that all the proposals would be handed over to Members but had not yet been received.
Mr Hughes said that overtime is voluntary and therefore cannot be considered as industrial activities.
He said that this issue could be resolved and the staff involved in the ban should not be punished.