Yes it is astronomical.

There is clearly going to be more than one view or emphasis on precisely what this astronomy is. This is my (dodgey) understanding of the views of Bradley, Burl and Ruggles.

Clava cairns have more of a SW orientation and are concerned with the midwinter sunset. The two passage graves at Balnuaran of Clava have very precise alignments on the sunset and, at one of them, Bradley in his investigations covered one of them in tarpaulin and witnessed the sunset as it might have originally happened.

RSCs have a much wider spread of orientations with an emphasis on the S - SSW arc. As Burl elegantly puts it: "Over 75% [of RSC alignments are] in those Styx-like regions where the sun and moon neither rose nor set"

The moon is seen as the focus of RSC alignments but not (at least in the general case) rising at one end of the recumbent, rolling along the top and setting at the other end.

Rather, the recumbent and flankers create a window on the horizon through which the moon, in its risen state, shines in on the observers.

Burl again: "An observer would have an arc of vision over 10 degrees wide. It would take the moon about an hour to pass across such an arc and this may have been what the users of the ring wanted in their ceremonies"

There are enough similarities between Clava cairns and RSCs to suggest a link but it looks as if the astronomical emphasis changed in the millennium or so between Clava cairns and RSCs.

Ruggles looked at the RSC alignments in much more detail but reached the same conclusions about the clear but general alignment on the moon.

To extend the discussion a bit, the Irish have axial stone circles (ASCs) such as Drombeg which have many similarities (and many differences) with RSC. ASCs are also, I think, much more recent than RSCs.

Despite the solsticial alignment at Drombeg, Ruggles conclusion on ASCs was that they "bear no consistent relationship with any specific astronomical body or event"