Hamilton's Bridge
Well, Broome Bridge really, but naming a bridge after a famous mathematician is a much better idea than naming it after the director of a railway company.
This rather unprepossessing bridge across the railway and the Royal Canal is the sight where the famous graffitist^Wmathematician William Rowan Hamilton defaced a stone to record the formula for quaternion multiplication, which is a way of turning three-dimensional arithmetic into four dimensional arithmetic so that you can more easily perform manipulations on three-dimensional objects. It seems odd to throw in another dimension in order to make things simpler, but that can often be the way maths works! In fact this invention of 1843 is heavily used by modern computer games for manipulating the 3D virtualities that we roam in.
Sadly the bridge doesn't have Hamilton's original graffiti, but we can see that the local mathematicians are still hard at work - with paint rather than pen-knives - and the original graffiti has been memorialised in a plaque.
On October 16th of each year there is a commemorative walk from Dunsink Observatory to here, with talks by famous mathematicians, and I rather think I'd like to do that this year if I remember.