Well, Debbie, your husband is going to have a glorious birthday on St. Patrick's Day! Make sure you get some information on the celebration of that holy day in Dublin. You will want to be prepared.
My darling wife and I stayed last May at Ariel House, which is a highly rated B&B in the Ballsbridge area of Dublin, right on the edge of the embassy district. The breakfasts there are legendary, and it makes a good choice for a single hotel stay in Ireland.
You will be one block from a train station that goes into the city center or to the south along the east coast of Ireland. This opens up a few opportunities for day trips out of town to places like Wexford, etc.
The Ariel House is also withing walking distance of downtown Dublin, perhaps a 30 minute walk to Trinity College.
For castles, you can take a trip down to Cashel on a bus. That's a fine place. Plan to include a visit to the Palace Bar on Fleet Street. It's a quiet, classic pub. There are more raucous places in Temple Bar. You can also pre-purchase a on/off bus tour, which is a fine way to get to know Dublin and visit some of the places that are a bit out of the way like Jamesons and Guiness and the sad old prison.
You can also take a bus up to Trim and the HIll of Tara. Trim has a divine castle and is a fun little town, and the Hill of Tara was the headquarters of the Irish Kings.
Best wishes for a delightful location. Dublin is full of many surprises and treats. You'll not be bored.