Herbert Park
Herbert park is a beautiful green space in Ballsbridge, on the southside of Dublin. It is bisected by a road with the same name:
On a walk around the park you're likely to find people playing all sorts of sports. On the north side there's tennis courts, a bowling green, and a children's playground. On the south side there's large open pitches where people play soccer and other ball sports. The park really comes alive during weekends, specially on sundays when the food market is open. Make sure to visit the pond that's always full of ducks!
The park used to be closed at night, but it's now open 24/7. However, please note that there is no artifical lighting inside, so it's not recommended to visit after dark.
The cache
On an otherwise unremarkable edge of the park, there's a long trail where people often walk their dogs. Head over there to find this cache:
Go find the Three-Pronged Tree on the side of this trail and stand next to it. The cache is hidden very close to it, but not on the tree itself. You should spot it soon enough.
The cache is small but not too small; plenty of space for trackables or other small goodies. A pencil is included along with the logbook, but if you prefer to sign it with a pen, bring your own. Feel free to take something, and leave something in return 
. Good hunting!
CO February 2022 update
A new fence has been installed in Herbert Park, closing off a small area where the cache is located.
I'm considering what to do about this. The cache is still accessible, but more stealth and effort is required. I've updated the difficulty accordingly.