UPDATE ** UPDATE ** UPDATE 2021-05-17
I've been told by the guys who run a ferry to and around Ireland's Eye that they are no longer landing people on the island due to the two landing spots being dangerous and are waiting for them to be repaired/enhanced. So, unless you know somebody with a boat, you won't be able to claim a find. I'll keep an ear (and an eye) out for any news that changes the situation.
Ireland's Eye is an island about 1.2 miles (2 K) off Howth - a peninsula to the east of Dublin. In past years it was actually a leper colony! From Howth you can see the island's most prominent feature which is a martello tower. Less obvious is the remains of an 8th century church. There is ample parking areas although you might want to get to them early as Howth is a very popular seaside resort. The first pier you will see (just after the train station) is known as the west pier and is generally used by the fishing trawlers. You should turn left here and try to get parking in the car parks on the right.
Walk down the pier, past all the fish shops and eateries, until you get to Ireland's Eye Ferries (N53 23.550 W6 04.07). You should book your passage online at irelandseyferries.com. They have two boats - one that goes around the island and the other that drops you off and picks you up later. It's €20 and €10 for teens and €5 children under 12 (asof May 2021). As far as I can remember, children by law must wear a lifejacket
Major Update! The island's surface is made up of very dense fern growth and areas that are rocky and barren. Instinctively you will tend to walk on the easier ground to avoid having to hack your way through the ferns and the thorny stuff that wants to rip your clothes. Unfortunately, the seagulls also use these rocky and fern-free areas to lay their eggs and raise their young. If you walk anywhere near a nest the gulls will become aggressive. While you may get away unharmed and regard it as a bit of fun, other birds or rodents will have taken the opportunity to raid the unguarded nest and steal the eggs so, without you knowing it, you may have cost a gull her family. It is a bird sanctuary and there is concern being raised over visitor numbers and activity on the island and how it affects the birds' breeding. To mitigate the effects, pathways have been cleared through the fern growth to allow us explore the island without causing any effect on the birds so PLEASE KEEP TO THE PATHWAYS.
Fingal Co Co are monitoring at the moment (June/July 2017) and will be holding a public meeting for "interested parties" on the future of Ireland's Eye so try to respect the bird sanctuary. As far as I know there are a series of plans to manage the island to protect not only the fauna (mainly birds) but also the fauna where invasive species are being removed for example and also to restore a well that would have been used by personnel who manned the tower and by clergy associated with the church.
The cache has been updated although I do need to put in a proper Geocaching label inside the box. You can get to the cache directly from the Martello Tower or you can go to the extreme end of the island and get to it on the way back. The cache and the route to it are child-friendly but do be aware that fern in the autumn has spores that are considered to be carcinogenic.
More details can be found about Ireland's Eye through a quick search on Google.
Ireland's Eye Ferries 3 W Pier, Howth, Dublin 13, Ireland irelandseyeferries.com +353 86 077 3021. UPDATE 2021-05-06 I'm not sure if there is a ferry available to take people to the island. It may be limited to trips around the island.