Quote:
"The characters 128-159 are not used in ISO 8859-1 and Unicode"


From http://www.pemberley.com/janeinfo/latin1.html

Therefore $chr(159) is meaningless to mIRC, it should not display anything. The issue with the editbox might be an issue with font-linking and Windows handling things a little differently than mIRC's end, which is strictly UTF-8. As Khaled has said, what happens in the editbox is entirely defined by MS's Richedit control behaviour.

Latin Capital Letter Y with Diaeresis is U+0178: http://www.fileformat.info/info/unicode/char/0178/index.htm

Use //echo -a $chr(376) instead (376 is 0178 in base-10).