Since you are using isop on mIRC, mIRC will have to see the nick list and that you are an Operator.
Right after connected, but before joining any channels, run:
/raw CAP REQ :twitch.tv/membership
This will make the nicklist with operator status appear, but I think you will find it to be buggy, the list won’t function sometimes. This is because mIRC is trying to update it by running /name which doesn’t work the way it does on an normal IRC setup.
You could run:
/raw CAP REQ :twitch.tv/tags
…but then you won’t be using isop, you will have to check the tags on each username as they do the command. It’s not impossible and fairly easy if you know what you are doing. If you are a novice I forsee some stress in your future.
Some people have been known to query /mods to check for a moderator. You will want to do securely to prevent any abuse (not just an “isin” check because if my name is matt_thomas and someone realizes what you are doing, someone could join as att_thoma and use all the commands because it “isin” the results. 
)
I have even thought about querying the undocumented TMI API, but since it’s not documented and supported, it can change at anytime without any notice.
-As Fugiman would never say “…don’t use mIRC…”