Don't suppose you saw if the bullaun stone is still there?
There is a similar stone to the ones you mention at Caheravart eclesiastic enclosure. These groups of four stones are oddities - there are two sets at Ashtown near Roundwood. I think they're more agricultural than anything else. There's also one at the back of a ruined farm building near Boleycariggeen, but these are all close-set stones about 1.5m apart.
Kilnaraune is built on a little hillock-type-thing and so could easily be the site of an earlier monument. However, the archeos reckon that the four stones may have ' formed the hinge-stones/corner stones in some form of structure no longer in existence.' Hourihane & Hourihane 1979.