Well, it's fairly uncommon to write an application to control another application, especially if mIRC has built in scripts to do what you want to do already. To be honest the extra complexity of controlling mIRC from another application is probably about the same as just learning a bit of scripting and using that.
As for actually controlling the application, you have lots of options, which range from sending network messages, sending window messages via Win32, or even simulating moving the mouse and hitting keys. I don't think any of your options are both currently possible in Construct and reasonably simple (eg. moving the mouse automatically will be incredibly complicated and will break if the window layout is ever so slightly different).
You're using the wrong tool for the job really. Construct is a game creator.