That looks like a triumph arch , I know Dublin had one to mark the Duke of Wellington's (educated at Trinity college) victory over Napolean at Waterloo , not sure if that is it , but it would be strange to take down such an arch to make way for a temporary gate to mark the visit of a regining monarch.
Given that it looks to be after 1900 , I would have thought that it would be a triumph arch to mark the Boer war , however I believe that arch is Fusiliers arch which is beter known as Traitors gate at the entrance to Phionex Park. It could be a victory arch to mark the end of WW1 which could have been destroyed during the War of Independance or the resulting Civil War which would meant the structure would not have stood for very long.
It is a bit confusing as the style of the gate is VERY 1800's.

Edited 1 time by KD 191 23/09/2010 23:27:45.