We love going to Dun Laoghaire for a family outing.
On Sundays, we often go for a walk/scoot down the pier, followed by a wander around the farmer's market in the People's Park where we grab some lunch (kids love the hot dogs) and then the kids have a play in the playground.
During the week, I sometimes take them down the pier and then the kids have a play in the small playgound by the shops/restaurants beside the Pavillion Theatre (which often has cool things on for kids).
Other times, we park by the Dublin institution of Teddy's and scoot down towards Sandycove Beach and then walk around the forty foot and pop into the James Joyce tower for a clamber up the stairs and a look at the view from the top of the martello tower it is housed in. Kids love poking their noses into the rock pools too.