What to see in Ireland?

Check into some day trips out of Dublin and forget renting a car altogether! There are tons of day trips by train or small and large tour companies: Check out Irish Rail, and Over the Top Tours. Wicklow/Glendalough; Bru Na Boinne (Newgrange/Knowth); Howth. Good luck and have fun. Dublin is a fun city with lots to see and do. I've been twice and always allow at least 3 days in Dublin in addition to wherever else I am going. Cheers!

You can't "do" Ireland in 4 nights so I suggest you not even try. For this short time, I'd recommend seeing Newgrange and Knowth along with the Boyne River sites. Spend the first night there. Drive to the Wicklow Mountains and see Glendalough so you'll need to spend the second night there. Drive back to Dublin and drop your car off. Spend the remaining time in Dublin. If you're really intent on the Cliffs, you can drive straight to Doolin. There you could tour the Cliffs and maybe get in a short cruise. You could see Clonmacnoise either going or coming. Personally, I was not overwhelmed by the Cliffs.

In your short time I, too, would stay around Dublin. Ken makes good suggestions. Four nights really only gives you three full days, plus parts of two others, depending on your flight times. It isn't a tough drive straight across to Galway and down, though, if you really want to see the cliffs. I love them, and the west of Ireland is completely different in character than the east. If you look at www.viamichelin.com you can get an idea of how long it takes to get from one place to another (add about 20% to their time estimates). That might help you narrow down what you have time for.

In four days, I'd rule out the Cliffs of Moher and renting a car. Stay in Dublin, see the sights there and add a Bus Eireann organized daytrip or two to Newgrange and/or Glendalough (each seperate day trips). You should be able to stop by the bus station when you arrive in Dublin and pick up tickets for the tours you want. In May we bought ours the day before the tour. While in Dublin, there are a number of sights to choose from. Trinity College, Dublin Castle, the Post Office, Halfpenny ("hayp-knee") bridge, Kilmainham Gaol (jail), Churches, etc. depending on what interests you. I like the evening literary pub crawl too. Here's a site for day tours: http://www.buseireann.ie/inner.php?id=173#Day_Tours

To reinforce what others have said, I would stay in Dublin as well and day trip from there. There is plenty to fill four days in and around Dublin, but there's also a fast train to Belfast if you want to expand the experience a little.