Well it turns out it is none of these things. A quick call to the Dun Laoghaire Rathdown County Council revealed that in fact these colourful poles are actually purpose-built...
*** GEOCACHING POLES ***
...that were imported by the Council from Geocaching-HQ in Seattle as a means of promoting the game abroad.
And it so happens that with every set of "promotional poles" dispatched around the world by Groundspeak, well there is a cache hidden amongst them as well...! A bonus if you like.
Go figure
NOTE: Access to GZ is across the bridge from the Leopardstown side of the motorway along the pedestrian sidewalk on the right side of the slip road. There is easy parking near the ESB Main Gates (on weekends)... or use the nearby Bewleys Hotel.