This indicates an attack attempt to exploit a Buffer Overflow vulnerability in some IRC clients.
The vulnerability is due to an input validation error in the application when handling a malformed PRIVMSG message. A remote attacker may be able to exploit this to execute arbitrary code within the context of the application.
Trillian version .73 and .74.
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply appropriate patch or upgrade to non vulnerable version from the vendor.
Or Do not connect to untrusted IRC servers.