I've not been now for about two and half years (longest gap ever), but last I heard /some/ of Carnacs main alignements were open in the off season. I've never seen the Kermario alignements open and the three main sites are fully closed from about Easter-late October.
That said, it should never put anyone off visiting as there are literally hundreds of fully accessible, well preseverved and virtually unvisited sites in the area.
One day I'll write my book...