Stand up MMS style. There are different kinds. Wrestler? Muay Thai? The types of stances in MMS very widely.
McGregors normal stance is very upright, hips forward, and he kind of lurches when he punches (this is all from memory. I haven't looked or care to look carefully). He keeps his hands lower than a boxer to defend take downs, and keeps his legs closer together to help with kicks muay thai style.
In this stance, McGregor will not be able to generate enough speed or power to contend with Floyd even of he was a good boxer.
His only chance is not to fight MMa style in my opinion. It's to use his length early in the fight to try for a lucky hit while Floyd is cocky.