Hmmm in the limited range of my experience, I seem to have visited most of the exceptions and very few of the 'rule'! Dont know if I should concede the ones facing outcrops though, maybe if they can be interepreted as macro-hills :)

Ahaglaslin... Dont know if I can concede that one either, its not facing either up or down the hill!
Brennanstown/Labby Rock... follow the line of the valley with hills on either side so are niether uphill or downhill when they easily could have been.

Been to Kiltiernan, Legananny, Browne's Hill which face downhill.

This simply demonstrates that I need to see more portals as soon as possible :)

Just goes to show though, relying on photos can be deceptive. I had seen lots of photos of Dun Ruadh and yet I fully expected it to be on flat ground, looking at the same photos now it seems so clear that its on a slope. Odd.