If you definitely know that this guy got your IP off of IRC, then tell an IRCop. I've been DDoS'd by somebody who got my IP on IRC before, I found out who it was because before my computer went crazy and crashed, zonealarm noted down most of the IP addresses, including that of one person who I had seen arguing with others about being a kiddie. I told an IRCop, who looked at the logs, had heard other people complain about the guy, and kicked him off the network.