The Pyramid of Dublin is located on the top of the Victoria Hill (a.k.a the Mapas Mount) near Mapas Obelisk in the Killiney Hill Park. It was built in 1852.
A view on Dublin from the top of the Victoria Hill:
A view on the Irish Sea:
There is a walking path to the top of the Victoria Hill from the DART station Dalkey. A map with walking directions you can get for free in the Dalkey Tawn Hall, which is located in the old Dalkey Castle on the Castle street:
As I said, the view at Victoria Hill is the most spectacular view of Dublin I could find. Thus being in Dublin don't miss the opportunity to visit this place, it worth all the time spent and all the efforts to reach the top on foot.