I think it would be best to search parish records for Carlow first, to try to establish family lines. RC records for the county are not online yet, so maybe try either the Carlow library option mentioned by Sonas, or try the NLI of FamilySearch (if available) microfilms.
Butler is quite a common surname in the south, so it's probably not that safe to assume your Ellen was born in Kilkenny, also 1800 is before the start of many of the RC registers - so a more definite location, or details of her parents, would probably be required to search for her.
Shane