301 is only ever received from /whois. You can gather the same info of this whois line from the /who command. You can generally gather this same information, without sending a server command, with mIRC's built-in Internal Address List (IAL), which remembers the information it sees about people. When someone joins a channel or speaks, their address is recorded to the IAL. Sending the command /who #channel or /ialfill #channel will populate the IAL with everyone's information, whether they have spoken or not.
$ial(nick|address) .nick .user .host .addr .mark .account .away .gecos .id