Yes, indeed it is difficult to state exactly what the orientation is with Proleek. I suppose there's a certain amount of subjectivity involved in deciding that, but it's certainly not east anyhow. There are three support stones in total, one to the east and two towards the west side. The following photo perhaps illustrates the alignment best:

http://www.mythicalireland.com/ancientsites/proleek/proleek-dolmen-8.jpg

Don't forget there is a local folk tale which says Proleek dolmen was carried there by a giant from a nearby mountain. This could be an indication of the intended alignment. Slieve Gullion is mentioned in lots of stories like this about standing stones and giants.