Derek,

a further thought.

Lugg henge http://www.themodernantiquarian.com/site/15699/lugg.html and http://www.megalithomania.com/show/site/1564/lugg.htm was excavated and found to be a multi-period site. Fourwinds says "Excavations also revealed that this was in fact a multi-phase monument. The henge is the final stage, but it originally consisted of a timber henge. This was followed by a brief period of habitation before the henge and mound were raised." So maybe Rathfalla is something along those lines. I briefly searched for the excavation report to no avail but you may have better luck.

regards