Up to 1,000 patients face cancellations of out-patient appointments and planned surgery at two regional hospitals this week following the breakdown of talks over hospital doctors' rosters.
The chief executive officer of the Labour Relations Commission, Mr Kieran Mulvey, warned after yesterday's talks that the dispute could spread to other hospitals by the end of the month.
The Irish Medical Organisation is balloting its members on a nationwide dispute.
From this morning, Waterford Regional Hospital and the Midland Regional Hospital, Tullamore, will be affected by three days of industrial action.
Last week, 275 people had out-patient appointments or surgery cancelled at the two hospitals.
The Irish Patients' Association said around 400 out-patient appointments and 116 planned surgical procedures could be cancelled at Midland Regional during the three-day strike.
Seven elective surgical procedures due to be carried out yesterday, and which would have required overnight stays, were cancelled yesterday.
It had been hoped that further disruption could be averted if yesterday's talks at the Labour Relations Commission between the Irish Medical Organisation and the South Eastern Health Board resolved the dispute.
However, the attempt, which followed a weekend invitation by the LRC, quickly failed.
The dispute centres on changes in rosters at a number of hospitals including Waterford and Tullamore.