It is a bit out of the way, but if you're heading over to the Guinness storehouse or Kilmainham, it's not that far.
It is rather big, we went there this summer and only saw a small part of it. Our kids wanted to see some pick-up games of soccer, cricket, and hurling, which you can see on Sundays (although there weren't any hurling games we could find). You can also rent bikes there.
As far as wildlife, there's red deer near the president's house, and a zoo near the entrance.
The hop on/hop off buses go through the park. We weren't using these, we had a large family group, but it was easy to get to using the regular bus system.