Bench Mark on the tower of St Johns Church

 The benchmark was placed there in 1891 when the street was still called Nelson Street. It is known as an OBM as it is one of the Original (Old) Benchmark to distinguish it from the later Malin Head Benchmarks The OBM has a height of 29.3 Feet above Poolbeg Lighthouse in Dublin where all heights were referenced to before the OSI switched to Malin Head Pier in the 1960’s as that would give a better reading of mean sea level due to the increase of traffic in Dublin Harbour.