As of this time 04.2018 Iris is free trial with option to invite friends to use it for free.
There is also 1 time payment option to get it lifelong.
These pages are dedicated to these things
Last year 2017 Iris was freemium with the option to unlock all features by buying the Pro version. This means that you can use Iris for free forever but with limited options.
You can find the last freemium version here for Windows
And here for macOS
The first versions of Iris was totally free but I don’t remember which version number they were. You can try to find them from here
Note that I improve Iris a lot all the time and they may be a little buggy and not user-friendly because this was the beginning of this project, but they still work 🙂