The victim, named as medical student Matthew Sheridan, sustained serious burns in the incident at the Halo nightclub in Galway.
Mr Sheridan was taken to University College Hospital Galway and was later transferred to St James’s Hospital, Dublin, where he remained last night in a critical condition.
Gardaí at Mill Street Station in Galway are now investigating how the man’s costume caught fire.
It is understood they have spoken to a man who presented himself at the station yesterday morning.
Elsewhere, in Tallaght, Co Dublin, two other people were injured in firework and bonfire-related accidents.
A 12-year-old boy was left with second-degree burns after a firework exploded in his face. Separately, a woman in her 20s sustained burn injuries following a small explosion at a bonfire.
In another incident in Dublin, an ambulance was stoned by a group of youths as paramedics returned to Beaumont Hospital with a patient.
Cork City Fire Services reported a quiet Halloween night, while a Dublin Fire Brigade spokesman said there had been no noticeable increase in the number of calls it received.
The Fire Brigade processed about 1,200 calls in the Leinster region between 4pm, Wednesday, and 8am yesterday morning.
In Dublin alone, there was 650 fire calls. Of those around 190 were attended by the Fire Brigade. It reported that in a number of incidents, not only were cars set on fire but also driven onto bonfires.
“The majority of those calls across Leinster were related to bonfires, fireworks, and a number of car fires,” said the spokesman.
He added there had been an even spread in calls across Dublin City and the whole county.