We walked from ChristChurch to Guinness Storehouse....about 15-20 minute brisk walk. Doable, but with a 5 year old? We have a 6 year old grand-daughter....and I just don't know if she could/would do it? Does the 5 year old have lots of stamina? The city bus system is pretty inexpensive, 1.6 to 1.8 Euros average per ride. We rode the bus back to Trinity. Another possiblilty is one of the 2 (green or red) hop on hop off bus systems. They run quite often and would be a good way to get around more easily with a 5 year old. There is a route for the green hoho bus with stops available on the net: www.dublinsightseeing.ie/citytour.aspx
I too would recommend the hopper bus. How much you fit in depends on how early you start. I suggest seeing your most important "must sees" first. Then if you don't get to the others you won't be disappointed. Play it by ear as to whether you eat dinner in the city or not.