Irishman remains in a critical condition

An Irishman who went missing for five days in Sydney is still fighting for his life in hospital and has undergone more surgery.

Donal O'Sullivan was found naked at the bottom of a stairwell in a commercial building in Bondi Junction on Wednesday night, having last been seen leaving a nearby pub on Saturday morning.

A cleaner found the 33-year-old carpenter lying in a pool of blood before he was taken to St Vincent's Hospital with serious head injuries.

A hospital spokesman on Friday evening said that Mr O'Sullivan remained in a critical but stable condition.

He has undergone a series of surgeries to prevent bleeding and relieve pressure around his brain and underwent further procedures on Friday morning.

It's understood members of Mr O'Sullivan's family have flown to Sydney to be at his bedside.

Messages of support have continued to pour in from around the world on a social media site set up to help find the Irishman.

A mass was held at St Patrick's Church on Bondi on Thursday night, with attendees praying for his recovery.