I think St. Annes is my all time favourite place to bring the kids on Dublin Northside. Its huge - lots of it is flat so its a wonderful place to teach kids how to ride a bike or rollerskate. The playground is aimed at the under 7s, but there are lots of stationary wooden figures in it such as a massive Viking ship, plane, train, horses etc which even the slightly older kids get a kick out of. Outside the playground there is a purpose built picnic area which tends to be busy at all times of year filled with parents & minders braving the insane Irish weather.
In the last couple of years the park has added a massive climbing frame, zip-line and enormous swinging moustache for the bigger kids.
ON Saturdays there is a farmers market in the park, where you can get all sorts of food, plants etc etc. There is also a café in the Red Stables and the food is pretty tasty & not heavily processed - I can recommend their soup. Have fun and enjoy.