For montague's posted code to work in mIRC 7.1, change it to...

/alias ud say $utfdecode($regsubex($replacex($regsubex($lower($1-),/(.)/g,$mid(\A,-\n,1)),a,C9 90,b,q,c,C9 94,d,p,e,C7 9D,f,C9 9F,g,C6 83,h,C9 A5,i,C4 B1,j,C9 BE,k,CA 9E,l,CA 83,m,C9 AF,n,u,p,d,q,b,r,C9 B9,t,CA 87,u,n,v,CA 8C,w,CA 8D,y,CA 8E),/([A-F\d]{2}) ([A-F\d]{2})/g,$chr($base(\1,16,10)) $+ $chr($base(\2,16,10))))

Then, in mIRC, alt+o (or Tools, Options), Other, Lock... Untick Decode (at the right). Hit OK.

Else, you will have to recode the thing. And notice the 4th ")" at the end of the code; it is needed.