Pure random speculation, but if: http://www.megalithomania.com/show/image/57
is sandstone, could it be that it was marked out to make a rectagular trough, but then split in half before the trough was started to be hollowed out? It sort ofloks like there are marks along the bottom edge of the stone that could be the same as the ones on the upper part, but snapped off. Overall they'd make a vague rectangle.
Yep. That's one of the dodgey ones. It being displaced doesn't help its cause much either. However, there are at least four other stones at Castleruddery with similar marks and two of them are in situ ... the marks go down the side of the stone and must have been done before the stone was erected.