Gardaí are appealing for help in locating 55-year-old John Byrne, who is missing from his home at Davitt House, Driminagh, Dublin 12.
He was last seen at 10am on St Stephen's day outside St James' Hospital, Dublin 8.
John is described as being 5 foot 8 in height, of very thin build with dark hair going grey.
When last seen he was wearing navy slippers, navy pyjama bottoms and a navy jumper.
Anyone with information is asked to contact Crumlin garda station on 01 666 6200.