Most of the Cork circles have a NE-SW axis, so you can pretty much take your pick down there. Kealkil's one of the odd ones out I think, but it'd be pretty spectacular from there anyway!
Another candidate for a very clear day would be Altar wedge tomb where the sun might just set on the top of Mizzen Head.
It just dawned on me that sunset is about 4pm and I finish work at 6pm, doh!