This plaque commemorates the discovery of the quaternion formula (i2=j2=k2=ijk=-1) by Sir William Rowan Hamilton on 16th October 1843, as he walked along the canal from Dunsink on his way to a meeting in Dublin. Without paper to hand he scratched the formula into one of the bridge’s stones.
There is a commemorative Hamilton Walk organised by the Maynooth University Maths department every year. There is also a virtual version here.
For a Storify of Hamilton Day 2015 and the 25th Hamilton Walk, click here.