Looks like you have some extra closing braces. Also, you don't need $mircdir, if you do use it and $mircdir has any spaces in it (or subfolders in the path have spaces) it will need to be quoted $+(",$mircdir,system\dlls\mircustom.dll").
Code:
on *:start:{
  dll system\dlls\mircustom.dll SetIcon -m mirc-multi.ico
  dll system\dlls\mircustom.dll Titlebar -m AM %ver $status $fulldate
}