Top places/things to see:

Chester Beatty Library
Dublin Castle, Dame Street, Dublin 2
www.cbl.ie
Amazing Asian culture collection and best secret city garden.

The deer in Phoenix Park.

Dun Laoghaire Harbour (see picture)
There is nothing quite like a 99 ice cream on the pier!

St. Kevin's Park
Off Camden Row, Dublin 8
Apparently the most haunted graveyard in Dublin, but super pretty and covered in cherry blossom in the spring.