Original census forms for 1901 have survived in their entirety and contain a wealth of information for your Irish ancestor search. They are freely available online at the National Archives of Ireland's website. In addition, some regional heritage centres have photocopies of the returns for their local area which they allow personal callers to examine. Some charge a small fee. Others don't.
Each household across the island of Ireland was expected to complete its own Form A. This form recorded the names of all the individuals in the home on the night of 31st March 1901.
Enumerators, typically local police constables, went from house to house collecting the census forms from literate families, and filled in the required information for those heads of household who could not read and write. These census forms were signed by both the enumerator and the head of household, no matter who had filled in the required information.
All members of the household, plus any visitors who spent the night, were recorded by name, age, sex, relationship to the head of the household, religion, occupation, marital status, county of birth (unless born abroad, in which case only the country was recorded), their ability to read or write (or not) and whether they spoke Irish, Irish & English, or only English.
Forget any tender politically correct sensibilities, the census return also had to record whether the individual was 'deaf & dumb', 'dumb only', 'imbecile', 'idiot' or 'lunatic'.
Ages were often artificially rounded up or down to the nearest ten years. Many of the ages recorded were less than they should have been and should be treated with some caution. (If your ancestor search takes you to the 1911 census forms, you may find your ancestor has aged more than ten years in the intervening decade!)
Form B1 was the House & Building Return and it provides a useful overview of the buildings within the townland or street. More importantly, from a genealogical perspective, it provides an at-a-glance summary of all the heads of families in the neighbourhood, so it is possible you'll find relatives living nearby.
Form B1 also recorded the number of rooms occupied by each household; details of how the walls and roofs were constructed ie mud, stone, thatch; the number of rooms; the number of windows at the front of the house; how many families lived in each property; and the name of the landholder.
In addition, a grading of 1st, 2nd or 3rd-class house was awarded.
These Form B2s were officially entitled the 'Return of Out-Offices and Farm-Steadings'. They record the extra buildings or facilities attached or included with a dwelling place ie cow sheds, stables, barns, piggeries, forges, laundry, potato house etc. The enumerator was specifically instructed on the form not to include privies, ashpits, conservatories, greenhouses, summer houses and tea houses.
Form N was the Enumerator's Abstract on which he gave a summary of the total
number of houses within the townland or street, together with the total
numbers of occupants of each household and their religions.
Individuals living in institutions or on ships (in port) had to recorded on different forms.
Often, only the initials of the person were recorded, but you may find that additional information such as birth county, occupation and marital status will help you to identify some individuals with some degree of certainty.
These forms were: