First of all, it might be a nice idea to explore the village of Portmarnock itself.
The historical "heritage" town has a very polished and affluent feel to it, with the concentration on the large marina rather than fishing activity.
There are a host of great places to see and things to do in "Dublin's Fair City" - a vibrant, friendly and culturally rich and diverse international metropolis of very manageable size.
Howth is an instantly likeable, laid-back coastal town that bustles with activity. There is a great mixture of fishing, shopping, tourism and village life.
Enjoy the scenery extended out before you embark on the journey from Dublin to County Wicklow and wander around Glendalough at a pace that suits you.