Gardaí have launched a major investigation after a prison officer, who was assaulted by a prisoner yesterday, died from a heart attack.

PJ Doran, a medical officer at Loughan House open prison in Blacklion, Co Cavan, was on duty at the jail yesterday morning when he was involved in a minor incident with a prisoner.

It is understood that he was pushed in the chest by the prisoner and looked unwell after the incident.

He was very shaken and the colour drained from his face so Doran's colleagues decided to take him to Sligo General Hospital to see a doctor.

He was on his way to the hospital in a colleague's car when he is believed to have suffered a massive heart attack.

He was declared dead on arrival at Sligo General, which is a car journey of approximately 40 minutes.

Gardaí from Monaghan entered the prison soon after and are understood to have interviewed the dead man's colleagues as well as the prisoner who was involved in the altercation with the officer.

Although Doran died of natural causes, officers will await the result of a post mortem ? which will take place tomorrow - before deciding on how the investigation should proceed.

It is likely at this stage that the prisoner will be arrested for questioning on suspicion of assault and a file will be sent to the Director of Public Prosecutions.

A spokesman for the Irish Prison Service last night confirmed that an incident occurred at the prison and that an employee later passed away. He declined to comment on the allegation of an assault prior to Doran's heart attack.

Colleagues say that PJ Doran was a very popular member of staff at Loughan House and was heavily involved with the Prison Officers Association. He was married with three children.

Loughan House is an open prison housing 160 low risk inmates who pose little or no risk.