re Historic Societies etc - I dont know of any first hand, but Co. Meath is particularly rich in history back to prehistoric times, but also many ecclesiastical ruins, so I'd say there are a number of groups interested in the area (although maybe not based there). Definitely worth trying a few searches for some of these. Probably more general than just Stackallen - so maybe look for Slane, the Boyne valley or Navan, as well as Co. Meath in general.
I think the church in Slane is a good place to start - if they dont have anything themselves maybe they can put you in contact with the right group or author.
I'll keep an eye out for any images of the church in books etc
S.