It is thought the hitman panicked and made a poor attempt at disposing of the weapon in a wheelie bin and the getaway bike in a nearby street after realising he had shot the wrong man.
Gardaí said: “A number of items have been found that we are currently examining as part of the investigation.”
Mr O’Rourke, a 25-year-old homeless man who had been using emergency accommodation services, died of multiple gunshot wounds after being mistaken for a gang figure at Sheriff Street in Dublin on Thursday.
The intended victim is believed to have been Keith Murtagh, an associate of the Hutch gang which is engaged in a bloody feud with the Kinahan drugs gang.
Murtagh, 32, was only released from prison in the latter half of last year after serving a lengthy sentence for his role in the attempted robbery of a security van during which gardaí shot both him and his accomplice, his accomplice fatally.
Mr O’Rourke, who was a father of three, had a history of drug abuse and had been in prison for minor offences but campaigner for the homeless, Fr Peter McVerry, described him as a “harmless, lovable young fellow”.
He had used the drop-in centre run by the Fr McVerry Trust and the priest said he had tried to address his drug problem, managing to get clean a few years ago before relapsing.
Fr McVerry told RTÉ the latest shooting — the fifth in the feud — is a very worrying development as there seems no chance of mediating between the gangs.
“This particular gang feud is especially dangerous because it has become extremely personal. It’s not about money, it’s not about gang territory. It’s become very personal and it doesn’t seem that there’s any way of stopping it.”