The nurses' strike will affect 15,000 outpatients whose clinic appointment will be cancelled, the HSE said yesterday.
Thousands more patients will have their surgery cancelled if the threatened stoppage goes ahead on Wednesday.
The scale of disruption came as nurses' unions, the Irish Nurses and Midwives Organisation (INMO) and the Psychiatric Nurses Association (PNA) met with the HSE for several hours again yesterday at the Workplace Relations Commission in a bid to resolve the dispute. They will re-convene for talks this afternoon.
"Outpatients are estimated at about 15,000 people in a day so it's the biggest single number impacted," said Liam Woods of the HSE.
Hospitals continued to contact patients yesterday to tell them of the cancellations if the strike goes ahead.
INMO general secretary Phil Ní Sheaghdha warned that community services would also be hit. Public health nurses and community general nurses will provide only an emergency service.
She said yesterday that the unions had comprehensive internal discussions following the first day's hearing on Wednesday, but they remained strong that there must be a pay rise to address the issues of recruitment and retention.
"We have to look at the conditions nurses and midwives are working under and we also have to look at their remuneration. Otherwise we are going to continue with the cycle of constant recruitment but no net increase in your workforce," she said.
Maternity hospitals will also be affected by the strike.
All outpatient clinics will be cancelled as well as other activities. Labour wards and neonatal intensive care units will continue to be staffed.