According to An Garda Síochána, the man, who was in his 30s, was found unresponsive in a cell at a Garda station in Dublin.
Garda members initially attempted to revive him before medical personnel and emergency responders took over. Despite these efforts, the man was pronounced dead at the scene.
The coroner has been informed of the death.
In a statement, Fiosrú confirmed it had received a referral from An Garda Síochána in line with standard Garda procedures relating to deaths in custody. Investigators from the ombudsman's call team attended the station and began inquiries.
Fiosrú said it would not be making any further comment while the investigation remains ongoing.
