rcirc-coding-system-alist

rcirc-coding-system-alist is a variable defined in `rcirc.el'.
Its value is

nil

Documentation:
Alist to decide a coding system to use for a channel I/O operation.
The format is ((PATTERN . VAL) ...).
PATTERN is either a string or a cons of strings.
If PATTERN is a string, it is used to match a target.
If PATTERN is a cons of strings, the car part is used to match a
target, and the cdr part is used to match a server.
VAL is either a coding system or a cons of coding systems.
If VAL is a coding system, it is used for both decoding and encoding
messages.
If VAL is a cons of coding systems, the car part is used for decoding,
and the cdr part is used for encoding.

You can customize this variable.