The simplest way is to open up the Windows Character Map (you can type /run charmap from mIRC).
Character Map will only list the glyphs (characters) available for the selected font, so make sure you select something like Arial or Times New Roman which have a wide range of characters available. You should then be able to see a list of all available characters for that font, and you can select the ones you want to copy to the clipboard and paste them into mIRC.
Before you try pasting into mIRC, open up the font dialog for the window you want to paste into (type /font) and make sure the UTF-8 dropdown is set to Display and encode. If it isn't then the text will be garbled when you send the text
