I published a piece in Proceedings of the Royal Irish Academy (Vol 102C, Number 1, 2002), which I am told is on the internet somewhere, showing (in my view) that the people stood around the Oratory, different categories (monachi, penitentes ...) often on different levels/terraces; that even the clergy only entered the oratory at the Offertory.
I suggested that perhaps the 'mass stones' of the penal period were a continuation of the same culture ... the idea that there were British troops with eagle eyes on every hill in Ireland throughout the 'penal' period seems to me improbable. Presumably some sort of shelter was erected around the mass stone.
The very extensive chants 'covering' the Communio in the Stowe Missal would suggest that this was, in fact, an extremely long part of the service.
I'm not sure about the Mass rocks: does anyone out there know more about them and how they were used during the penal era? That the early Irish were far happier out of doors than inside is commonly understood, and no doubt they were happier attending Mass in such a location than their English co-religionists might have been.