Gardaí have tasked volunteers with the Cork Missing Persons Search and Recovery group to search the south channel of the river Lee as they issued an appeal for help tracing Cormac Ryan, 31, who was last seen on Sunday.
Two divers are in the water, two shore crews are monitoring the river surface from the quays, and a boat with side-scanning sonar is sweeping the river between Parliament bridge and the Port of Cork.
Gardaí have appealed for help tracing Mr Ryan, who has been missing from Cork City since Sunday.
Gardaí and Cormac's family are concerned for his welfare.
Mr Ryan is described as being approximately 5'11 in height with a stocky build, brown hair and brown eyes.
He was last seen wearing a dark green coat, blue jeans, wine coloured shirt and black shoes with a thick white sole.
Anyone with information is asked to contact Anglesea Street Garda Station on 021 4522000.