A contractor has been killed almost 2km (1.2 miles) underground as he worked on the controversial Corrib gas tunnel.
Gardai and the Health and Safety Authority (HSA) have launched separate investigations into the accident, which happened under Sruwaddacon Bay near Aughoose in north Mayo.
The German man, who was single and aged in his 20s, was part of the maintenance support crew employed by Herrenknecht, the manufacturers of the tunnel-boring machine.
He sustained fatal head injuries in the accident just before 9am at the font end of the Corrib tunnel 1.8km (1.1 miles) underground.
Shell E&P Ireland Limited (SEPIL) expressed its sincere condolences to the family and co-workers of the man.
"Construction work on the tunnel has been suspended," it said.
"In the coming days, SEPIL will provide support to staff and contractors, who are so deeply shocked and saddened by today's tragic incident."
The victim had worked on and off site since November 2012.
He was pronounced dead at the scene and his body has since been brought to Castlebar Hospital for a post-mortem examination.
Joint contractors - BAM Civil and Wayss & Freytag - were appointed by Shell Ireland to build the 4.9km (three mile) Corrib tunnel which will eventually run under the bay to the Bellanaboy Gas Processing Terminal.
In a statement, the contractors said operations were immediately suspended at the tunnelling site and a full co-ordinated investigation has been launched
"The joint venture is co-operating fully with the Garda and Health and Safety Authority investigations which are under way at the site," it said.
"The management and staff of Wayss & Freytag and BAM Civil express their sincere condolences to the family of the deceased, whose details will be released once all his next of kin have been informed."
The controversial Corrib gas field lies about 80 kilometres (50 miles) off the north-west coast of Ireland and the onshore pipeline works is the final phase of the scheme to be completed.
The specialist tunnel boring machine, which was imported last summer, has an an internal diameter of 3.5m (11ft 5in). When the tunnel is constructed and the machine removed, the 20in gas pipeline will be installed.
The project is expected to be completed in late 2014.