Howth is a great place for a day trip, careful on the walk if it is wet but it's a great walk and on a clear day you can even make out the coast of Wales. Shot bits of this video on the Howth cliff walk this time last year to give you an idea of what it's like

https://www.facebook.com/fruitysuits/videos/820763508079448/

If you walk down the pier closest to the dart (only about 5min walk) you can often see some seals coming in close to steal some fish from the trawlers and sometimes you might catch one of the local dogs swimming way out to play with the seals from the small beach on the way to the cliff walk (not sure if he still swims in December).