One workman has died and another was seriously ill after being overcome by fumes while working in a sewer.
The men, employees of the drainage and cleaning company DrainTech, were working up to 20ft below ground in a housing estate in Portmarnock, Co Dublin when the accident happened.
They are believed to be in their 40s.
Authorities confirmed two men were taken from the sewer, one of whom was pronounced dead at Beaumont Hospital while the second was said to be in a serious condition.
Three members of Dublin Fire Brigade were treated following the rescue efforts, two as a precaution for noxious gases and one who suffered a minor injury in the operation.
The Health and Safety Authority said it originally received reports of a double fatality at the scene but later confirmed only one of the workmen had been pronounced dead.
The Dublin Fire Brigade said it was alerted to the accident at 2.30pm and dispatched three appliances, three ambulances and an advanced paramedic unit to the suburb.
The sewer was located in a residential area of Portmarnock called Drumnigh Woods.
Two inspectors from the HSA were also sent to the scene and an investigation has been launched into the circumstances surrounding the deaths by the Garda and the HSA.
The inquiries are centring on whether the men inhaled lethal fumes while working in underground pipes and sewerage systems.
Alan Farrell, local Fine Gael TD, extended his condolences on social media.
"My thoughts with those involved in an industrial accident in Portmarnock this afternoon," he said.
Fire crews reported the difficulty of the conditions that they were working in, a confined space, below ground and at risk of dangerous gases.
The DrainTech company operates mostly in the Dublin region and has been in business for more than 20 years offering services from unblocking sinks and toilets to CCTV camera inspections, clearing septic tanks and grease traps with staff qualified in confined space and deep manhole entry.