OSI Benchmarks

OSI benchmarks. Image reproduced with permission from Dr. Catherine Porter, author of this RTÉ article.

There are several of these located around the town. I'm not sure which are still in existence however. The marks, resembling an arrow, were chiselled into masonry by OSI surveyors when the country was surveyed in the early 19th century to create the first detailed and accurate maps. Typically, they're located on walls and gate pillars. There should be one located on the bridge in the town and a second one on a gate pillar on New Abbey Road. (At the gate on the left hand side of the road, just before the turn for McGarry's Lane when heading towards the cemetery.) A benchmark corresponded to an elevation in feet above a sea level datum (the reference point located at Poolbeg Lighthouse). The elevation was indicated on the 25" map with a crow's foot symbol. Surveyors inserted an angle iron into a socket located near the mark, and this was used as a bench for resting a levelling rod on during the elevation measurement process.
This recent article on the RTÉ website by geographer Dr Catherine Porter and Margaret Sullivan, University of Limerick, gives a background on benchmarks and their use use in the surveying process.

Image reproduced with permission from Dr. Catherine Porter, author of the RTÉ article.