Again using DDE, this time the sender is the mIRC in the channel you want to monitor.
As in Onlines example, using dde names "mIRC1" (receiving) and "mIRC2" (sending)
Code:
on *:join:#channel:{
  dde mIRC1 command "" /announcejoin %announce1 %announc2 %announc3 
}


In the receiving client
Code:
alias announcejoin {
 
  echo -a * mIRC2 sent: $1-
 
}