ircu is the name of the IRC daemon software written for and used on the Undernet

Undernet

The Undernet is the fourth-largest publicly-monitored Internet Relay Chat network circa 2011, with about 12 client servers serving 60,000 users in 11,000 channels at any given time....

 IRC network.

ircu is one of the few servers that is based on the irc2.7 codebase of the original IRCd

IRCd

An IRCd, short for Internet Relay Chat daemon, is server software that implements the IRC protocol, enabling people to talk to each other via the Internet . It is distinct from an IRC bot that connects outbound to an IRC channel.The server listens to connections from IRC clients on a set of TCP ports...

.

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 protocol P10.

Several other IRC daemons are derived from ircu, including but not limited to Nefarious, Asuka, snircd (used by QuakeNet

QuakeNet

QuakeNet is currently the largest Internet Relay Chat network, with a peak average of about 135,000 users and 100,000 channels. The network was founded in 1997 by Garfield and Oli as a new home for their respective countries' Quake channels...

) and OpenIRCd. DreamForge (formerly used by DALnet

DALnet

DALnet is an Internet Relay Chat network that is currently populated by a stable population of around 30,000 users in about 18,000 channels, with 40 servers making up the network....

) is also based on ircu, but it was forked early during the development of ircu, hence there is little resemblance.