Hi Mary,
I live in Tralee, which is 60 kilometres from Kenmare. I grew up in Kenmare and my parents still live there. My brother actually lives in Claddanure. I know "Jacko Dandy" and his eldest son Patrick; I never heard of there being any family relationship between "my" Sullivans and the Sullivan Dandys - there are several different Sullivan and O'Sullivan families in Kenmare.
It is indeed difficult to get information on births etc. in Ireland, as Government Birth Records only go back to 1865. I'm sure that you must have visited the General Registrar's Office at Lombard Street while you were in Dublin. Prior to 1865, only Church Records are available and these go back to the early 1800s at best. The quality of Church Records varies from place to place.
I have managed to get some details to about 1800 but I'm sure that I will not get back any further. Do not be disappointed if your search ends with Denis, born 1809!!
Census information from 1901 and 1911 is available, but all census data prior to 1901 was destroyed by fire.
COLM