I lived there for 6 years and really liked it.
The houses are lovely, the rooms have high ceilings and if you are lucky, the original fireplaces are really nice.
I used to walk to the bus stop and the local shop at night regularly and never felt threathened or unsafe.
The whole area is a little "rough around the edges" as horusd has put it, but if you are looking in Dublin 8, then that will come as no surprise. Kilmainham is very quiet compared to Dolphins Barn, or the Coombe.
Feel free to PM me if you have other specific questions.