Definitely only access from the shore, and at low tide. I had a walk a good way along the clifftop path which heads south from Stairhaven, and it is either precipitous or a mass of bramble and gorse. Ouch. Nonetheless I recommend a walk along the path as there is a good view down to the broch at one point and it helps to show the sites place in the land and seascape. Of the brochs seen in west Galloway I thought this was certainly the best preserved, even though that's not saying much. Well worth a visit.. but do watch that tide.
We gained access by climbing from the shore below, only possible at low tide. The approach via the gully on the northern side is somewhat easier but not as exciting. It is maybe possible to climb down from the cliffs above but we didn't check this out. There is enough remaining of a substantial broch to make the visit worthwhile and is a beautiful place just to sit and consider the original builders (and the in-coming tide). It's not the best work I've seen but pretty neat nonetheless. There are no door checks or guard cells but there is some remaining intra-mural staircase. So it's a broch all right and quite rare for this part of Scotland.