From the "standard" set of RAW numerics located on mIRC.net
Quote:
Raw Numeric 481 (index)

Returned when a non-IRC operator attempts to use a command (such as KILL) that requires IRC operator status.
Syntax
481 :Permission Denied- You're not an IRC operator
Example
481 :Permission Denied- You're not an IRC operator
Supported By
All
Notes
Some servers may return this for certain variations of a normal command. For example, some networks will only IRC operators to access some STATS replies.