An awful lot of records were destroyed in the war of independence and civil war, but conversely the civil war pensions and IRA records from the same time are pretty useful.Found the Irish side of the family the most difficult to trace. My grandfather's sister either died at one year old in Limerick or in her sixties in Ballymena.
The census records are patchy, really until the foundation of free state after the war of independence.
Finding older records can be hard, unless the family were Church of Ireland, who kept good records of marriages and baptism.