6 days in london and 6 in ireland

My husband and I are going to London and Ireland on 9/11 - 9/28 this year. What would be the best things to do in such a shore period of time? To get to Ireland should we fly or take the ferry? We will fly to Dublin and from there what would be the best to see and spend our time? We will go to London first then to Ireland and back to London to catch our flight home. Thanks for the help!