If you've visited Parteen Weir, Blackwater Culvert, (maybe even Ardnacrusha power station itself), and still want to see more, then this might just be the place for you. If you're only in it for the cache, skip to the important bits about transport/parking etc.
Still with me? When Ardnacrusha hydroelectric power station was being contstructed (1925-1929), a narrow gauge railway was built to carry building materials from Limerick docks to the construction site. Although the majority of the line is now gone or buried, the section of track bed from Parteen to Ardnacrusha is still extant and is popular with locals for walking/jogging. If you look carefully at the start of the trailhead in Parteen you will see a piece of rail still in situ.
The cache is in a spot that has played a rather less significant part in the history of the power station. It is on an avenue, complete with street lamps that goes nowhere and apparently has no purpose. Looking at the aerial maps of the area reveals that the most likely explanation for its existance is that the avenue used to lead through to the ESB houses; this would have allowed high ranking employees at the time to drive between their home and the power station without having to go out onto the main road. I imagine that when the road stopped being used for this purpose it was blocked off from the houses and has since become the curious cul-de-sac you see now.
Important bits about transport/parking etc.
If you intend to drive and park at the Ardnacrusha end, please be sure to turn in at the waypoint given below (it is signposted for Ardnacrusha pitch and putt). One of the other nearby turn offs brings you down a road of private residences - please do not use this road or park on it. Locals going for a walk generally park just in front of the tennis courts. You can also take a bus from Arthur's Quay in Limerick city to the suggested parking spot at Ardnacrusha (http://www.buseireann.ie/timetables/1425903716-313.pdf) although it is quite a limited service. There is also a carpark available at the Parteen trailhead. Refer to the waypoints for more information.
The cache is a little off the avenue and under tree cover so GPS might not be too accurate. Contains a logbook, pencil, and some kid friendly swag. Please replace as found.