Emoticons in mIRC has been suggested and discussed (at great length) in the past. Personally, I think it's unlikely they will be added, but I guess the next major version, or the major version after that will reveal that to us.
You can see other threads
here and
here. You can also use the search feature, expand to 'Feature Suggestions', 'All Posts' and search for 'Emoticons'.
Regards,