mirc-options/irc
then click on "events", where you have the option of having joins be in-channel/in-status-window/hide

or, this suppresses the normal join event and you can script yourself a replacement message like so:

Code:
on ^*:JOIN:#:{
  if ($nick == $me) { echo 3 # * Entering # $time(yyyy/mm/dd HH:nn:ss) $chan(#).mode using $server $port }
  echo $color(join) # $timestamp * $nick $+($chr(40),$gettok($fulladdress,2-,33),$chr(41)) has joined # $iif($comchan($nick,0) isnum 2-,Shared Channels: $v1) $iif($ial($address($nick,2),0) isnum 2-,Clone: $v1 nicks)
haltdef
}