In relation to further sightseeing recommendations the best way to take in the city's numerous tourist attractions is to take the Hop On, Hop Off bus tour which by coincidence goes by The Merrion Hotel every 15 minutes and includes stops at Dublin Castle, Christchurch Cathedral, Trinity College (The Book of Kells), The Guinness Storehouse, Kilmainham Geol, The Jameson Distillery and Phoenix Park.
If you would like to go further a field I would highly recommend a visit to County Wicklow with the "Wild Wicklow Tours" who operate an informal but highly informative tour of Wicklow which begins in the celebrity filled suburbs of Dalkey and Killiney (home to Bono, Enya, Eddie Irvine, Lisa Stansfield to name but a few) and then goes on to visit The Avoca Woolen Mills, St Kevin’s Monastic Settlement at Glendalough, the beautiful villages of Laragh and Roundstone.
From there you trek up and around Wicklow and a specific area called The Sally Gap to fully take in the beauty and magnificence of the mountains and Lakeland scenery before heading back to Dublin via the Blessington lakes.
If you were interested in all things historical I would recommend a visit to Newgrange in the Boyne Valley. This site is 5000 years old and pre dates the Great Wall of China and The Pyramids of Egypt.
Newgrange is a Neolithic Ritual Centre and Passage Tomb with architectural links to the pre-historic maritime peoples of Portugal, Spain and the Western Isles. Also featured on the tour is the charming village of Slane, The Battle of the Boyne Site and The Hill of Tara, which is the ancient royal site of The Kings of Ireland.