Most visitors come for the abbey itself, a restored Cistercian church of considerable age and atmosphere, but running across the river beside it are the low, angular remains of structures that once made the site economically viable in a very practical way: fish weirs, built to trap the river's catch as it moved with the current.

A fish weir, in its simplest form, is a barrier or series of barriers placed across a river to funnel fish into a confined space where they can be netted or lifted out by hand. Stone weirs of this kind were a common feature of monastic economies in medieval Ireland, since religious communities needed reliable sources of food that could be eaten on days when meat was forbidden, and eels and river fish filled that role. The historical record here is unusually direct. The Civil Survey of 1654 to 1656, a detailed Cromwellian-era land assessment, describes the vicinity of Holycross Abbey as containing 'fower Eele fishings', suggesting that eel trapping in particular was a significant and recognised feature of the site at that time. Eels were a valued food source throughout medieval and early modern Ireland, and the language of the survey implies these were established, named fisheries rather than incidental arrangements. The first edition Ordnance Survey six-inch map of 1840 confirms that the weirs were still present and mappable nearly two centuries after the Civil Survey was compiled.