Ireland in 3 days?

Scroll down the To The North section and you will see a thread called "Ireland in Two Days". Lots of good suggestions were given there in the past few days.

You didn't say anything about your situation. Is this your first trip to Ireland? If not, where have you been. On the assumption this is your first visit, I'd suggest going to Dublin for the whole time. Look into a day trip to Newgrange if the neolithic tombs sound interesting. There's other sights on the tour but I consider these to be the best part. What parts of Great Britain have you seen? You might be better off seeing some places there.

Definitely Dublin and surrounds.
S :0)

Three days in Dublin would work nicely for you. There is enough to do in the city, but you can also take a nice day trip to places like Glendolough via CIE (the Irish national bus company).

CIE is a private tour company. The national bus company is Bus Eireann.

We will be in London for 5 days, and those days are booked with business and sightseeing. We thought since we were that close, we could maybe squeeze three more days into our trip and see what we could of Ireland before we had to return. With the comments, I think we will concentrate in Dublin. Can you take a ferry over, or would it be better to fly over?

You can get a ferry across Jane (from Holyhead in North Wales to Dublin) but far easier and quicker to fly, particularly if you're travelling from London. I think you are making a wise decision to concentrate on Dublin for the three days. Enjoy it!

Trying to take a ferry from London to Ireland would burn a full day. You can easily catch a short flight to Dublin and save yourself a lot of time and probably a lot of money.