A lot of IRC commands are transparently relayed to the server. However a large number are handled seperately to make it easier to use IRC.
The handled commands are found below while any not listed will be relayed to the server.
/notice $target $message
/join $channel [$key], [$channel [$key]]
/ctcp $target $ctcp_command [$ctcp_args]
/alias [$alias $rule]
/alias del $alias