Gardaí say the 22-year- old soldier, believed to be a private, suffered serious head injuries when he was attacked early yesterday morning in the Youghal, Co Cork, pub.
Superintendent Flor Horan, who is leading the Garda investigation, said the alleged assault took place at The Usual Place pub at South Main Street, Youghal shortly after 12.30am.
The soldier, who is understood to be single and based at Collins Barracks in Cork, was rushed by ambulance to Cork University Hospital where his condition deteriorated after arrival. Shortly before midday doctors were forced to carry out emergency brain surgery in an attempt to stabilise his condition.
Gardaí had been called to the pub shortly after the incident.
Supt Horan said they carried out a thorough forensic examination of the premises, especially the smoking room.
Detectives seized CCTV footage from the public house and are also looking at footage recorded in nearby premises.
Supt Horan said a large crowd was present in the pub and there could have been upwards of 20 people in its smoking room at the time of the attack.
Anyone with information is asked to contact Youghal garda station at 024-92200.