An inquest into the death of a Mayo man who died following a gun attack in Nigeria has returned a verdict of unlawful killing by persons or persons unknown.

In a rider to their verdict, jurors at Castlebar Coroner's Court said they felt security and safety of the construction company PW Nigeria's personnel should be its first priority.

The 11-member jury deliberated for less than ten minutes before arriving at the verdict.

The inquest heard evidence regarding the death of Robert Gray, who died following an armed attack almost two years ago.

He was shot by armed robbers in the Taraba area of the country on 23 August 2012.

He sustained serious injuries to his upper leg and died a short time later.

The 45-year-old engineer had been working for the PW Nigeria construction company.

He was being driven back to a camp where workers were accommodated when the vehicle he was travelling in was attacked.

While the matter has been investigated by the Nigerian authorities, nobody has been convicted of any offence.

Seven suspects were arrested after the incident and two were charged with murder.

However, on the instruction of the Nigerian Director of Public Prosecutions, a re-investigation of the case has been ordered.

When he was questioned by police in Nigeria, Mr Gray's driver Armayau Abubakar did not identify either of the two main suspects in an identity parade.

The case is still open but is not being investigated by police at present.

A statement from the Deputy Head of Mission at the Irish Embassy in Abuja, Aoife Ní Fhearghail, was read to the jury by Coroner John O'Dwyer.

Ms Ní Fhearghail said there has been no information provided by the Nigerian authorities since last October.

She has expressed the view that a conclusion to the investigation is unlikely any time soon.

Provision exists for an inquest to be held in Ireland, in the event that an Irish national dies abroad. No inquest into the death of Mr Gray was conducted in Nigeria.

In his testimony to the inquest, a former employee of PW Nigeria, Jim McKenna, said he believed threats had been made against Mr Gray in the weeks leading to his death. 

Mr McKenna arrived at the scene of the attack minutes after it happened. He said Nigeria was a country rampant with corruption and expressed the view that police officers were complicit in the killing.

He said he did not believe PW Nigeria took adequate care for the safety of its employees.

Under cross-examination by Christian Douglas, barrister for the company, Mr McKenna said he did not agree with police accounts of the events on the day of the murder.

He contends the ambush took place half an hour earlier than stated and that Mr Gray died in his car on the way to hospital, not following admission there.

In his evidence, PW Nigeria's Regional Manager John Coen said the company was aware of threatening messages on Mr Gray's mobile phone in October 2011.

The Irish Embassy in Nigeria was informed at the time. Mr Coen said he was subsequently advised that similar messages were sent to "countless" other people.

Under cross-examination by Liam Guider, solicitor for the Gray family, Mr Coen said the state of law and order in Taraba state had deteriorated in 2012.

He said before that things had been easier for workers but it became evident that enhanced precautions were needed from early 2012.

A memo was sent to all staff informing them that a curfew was being implemented between 7pm and 6am. This entailed them staying in the accommodation camp.

Staff were told they would be sent home if they failed to adhere to the restrictions.

In addition, steps were taken to have armed guards travel with staff members on journeys they made.

Mr Coen denied people were in a state of anxiety as a result.