These records can be searched online at the National Archives of Ireland website (and also for account holders on the Ancestry and FindMyPast websites). Both the original returns, and their transcriptions can be viewed FREE of charge.
Even if your ancestors had left Ireland before 1901, you may find other family members (of previous and subsequent generations) who remained in Ireland. You can also discover more about who stayed on in their townland, which may lead to the discovery of relatives still living in the area today.
Form A lists the personal details of each household. Form B1 will give you a detailed description of the building they occupied and and other out-buildings (e.g. size of farmyard). It will also lend a clue as to who owned the property and who their neighbors were.