ircu is one of the few servers that is based on the irc2.7 codebase of the original IRCd.
At the time the irc2.8 codebase was being forked due to abuse prevention, ircu implemented TS, or time stamping, which stored a unique time stamp with each channel or nickname on the network to decide which was the canonical one to keep. Later versions of ircu implemented the P10 protocol.
Several other IRC daemons are derived from ircu, including but not limited to Nefarious, Asuka, snircd (used by QuakeNet) and OpenIRCd. DreamForge (formerly used by DALnet) is also based on ircu, but it was forked early during the development of ircu, hence there is little resemblance.