Here are my Irish friend's recommendations of Dublin area things to do. I'm sure most are listed in the guide books:
"Things to do/see in Dublin in no particular order. Dublin is divided by the River Liffey that runs west to east into the Irish Sea. Dubliners refer to themselves as Northsiders or Southsiders depending on what side of the Liffey they hail from. Traditionally the South side was where the more wealthy folks lived but that has changed a bit over the years."
Guinness Brewery
http://www.guinness-storehouse.com/en/Index.aspx
~excellent self guided tour, free pint, and great view of the city from the top
Kilmainhan Gaol
http://www.heritageireland.ie/en/dublin/kilmainhamgaol/
~interesting insight into Ireland’s history and war of independence, this jail has also been used in several prison movies including the original Italian Job!
Garden of Remembrance
http://www.heritageireland.ie/en/Dublin/GardenofRemembrance/
~my favorite garden in Dublin, I always stop by there, beautiful sculpture of the Children of Lir
Trinity College
http://www.heritageisland.com/attractions/trinity-college-library-dublin/ and
http://www.tcd.ie/
~beautiful campus to stroll around, the Book of Kells is on view here, The Buttery was a really cool campus bar with some food, but I was disappointed to see it had been remodeled into a restaurant and lost its old world feel.
The Brazen Head
http://www.brazenhead.com/index.php
~oldest pub in Dublin dating back to 1198! Good pub food, live music, always an interesting people watching spot.
Temple Bar
http://www.visit-templebar.com/
~fun part of the city, great music, shops, restaurants & bars. Just off the quays, one of the older parts of the city. There’s always something happening here. In college I used to frequent the second-hand clothes shops here. Walk down Crown Alley towards the river and you end up at the Ha’penny Bridge.
Ha’penny Bridge
http://archiseek.com/2010/1816-hapenny-bridge-dublin/#.UDKHYqmPVas
~my favorite bridge in Dublin (we have a lot of bridges). This one is a foot bridge and is always full of people. The video for Thin Lizzy’s song ‘Old Town’ had Phil Lynott singing on this bridge.
Grafton Street
http://en.wikipedia.org/wiki/Grafton_Street
~pedestrian-only street on the south side, between Trinity & St. Stephen’s Green, funs shops and cafes, always plenty of buskers (street musicians), Glen Hansard, Rodrigo y Gabriela and Damien Rice started out on this street!
St. Stephens Green
http://en.wikipedia.org/wiki/St_Stephen's_Green
~ Pretty park in the city, always a great meeting place on the south side. On the north side we would always say… meet you in front of the GPO (General Post office)
General Post Office (GPO)
http://www.anpost.ie/AnPost/History+and+Heritage/History/GPO+Dublin/
~built back in the 1800s, this is still a working post office and a really cool building with quite a history. It was partially destroyed during the 1916 Rising (war for independence) and you can actually see some of the bullet holes in the huge columns outside.
National Museum of Ireland
http://www.museum.ie/en/homepage.aspx
~in particular the Decorative Arts & History building
http://www.museum.ie/en/intro/arts-and-history.aspx Great history of the country, beautiful artifacts.
Jameson Distillery Tour
https://www.jamesonwhiskey.com/us/visit-us
~the Old distillery is in Dublin and is a fun tour.