In Brennans book he claims the difference is only one and a half degree to the north, I've heard others say this is only one and a half to two diameters of the suns disc, Brennan says 'At Equinox the variation is so minimal that it need not be considered at all'.

Frank Prendergast has done a lot of work on alignments to solar events and more recently on other monuments, he recently claimed that only 10% of passage tombs in Ireland are potentially aligned on significant astronomical events. There's not a whole lot of coverage on Irish passage tombs in Clive Ruggles book unfortunately, though I did hear a rather bizarre claim that he regards concepts like the equinox as 'too abstract' for neolithic people. He does say in the book that dates for the equinoxes and cross-quarter days are meaningless unless you can get an exact date for the solstices, which he claims is near impossible using the structures he examines.
Brennan makes a very simple point in this regard, if you count the total days the suns shadow moves from one point to it's extreme, then back to it's original point and divide by two you should get the exact date.