The Book of Kells Experience, including the viewing of the Book of Kells and a visit to the Trinity College Long Room is an absolute MUST. Know that the Long Room is currently restoring their tons of shelves so it may look a little empty on the shelves. We highly recommend the 45 min guided tour-- they share some of the fun superstitions and ghost stories that the Irish are famous for.
St Stephen's Green is a lovely park located at the end of Grafton Street. Take a stroll around the busy shopping area, get some delicious coffee (because Dublin coffee shops are phenomenal) and take a lovely stroll in the park (weather allowing).
Didn't think we would forget the bookstores did you? There are some incredibly cute bookstores (and comic/game shops) around Dublin. Sharing a favorite here, but be sure to google them!
While we'll be getting married at Tinakilly, just a few minutes away is the cute town of Wicklow! Enjoy a walk to the lighthouse, get a cute coffee, and walk around!