Looking at other posts I am starting to form the view that only 1 day is needed to spend in both these cities? Assuming we spend 2 nights and allow one full day to look about?--
Any discussion?
I would give Dublin a day and a half, depending on what you want to see.
Belfast deserves a full day. Dublin a couple days.
I've been to Dublin twice, staying for a couple of days both times. I had a great time, and no shortage of interesting things to do. It really depends on what your interests are. Take a good look at the Dublin section in Rick's guidebook, and others. That will help you decide how much time you want to spend there. I don't have any personal experience with Belfast, but am planning on spending a day there this summer. Barb