One of the most comprehensive studies of Irish surname frequency was conducted in 1894 by Robert E. Matheson, who was Register General of Ireland at the time. His statistical report based on Irish births registered in 1890 was published as Special Report on the Surnames of Ireland, subtitled "With Notes as to Numerical Strength, Derivation, Ethnology and Distribution."

If you're interested in Irish surnames, this book is a great resource. It's available (free) online in several formats. Here's a link to the archived copy at the Cornell University Library: https://archive.org/stream/cu31924029805540#page/n3/mode/2up

By the 1890s, Matheson concluded 'Murphy' was the most common Irish surname - and it continues to be #1. Matheson's report notes that Irish surnames are derived from several broad classes. Later studies suggest the mnemonic "APOD" for the four classes of surname derivatives: ancestral, place, occupational and descriptive. An ancestral example is O'Neill, as descendants of Niall of the Nine Hostages. The surname Brody was originally derived from a place name meaning 'ditch' in Gaelic. My own surname (by marriage), Hickey, comes from O hIceadha-icidhe, meaning 'doctor' or 'healer'. An interesting name denoting a personal feature was Mac Dughghaill (modern MacDowell), meaning 'black stranger'.

The use of surnames began at different times, in different places. Generally the wealthy urban dwellers were the first to adopt surnames. Authorities note it was the norm in 11th century Europe not to have a surname but by the 1400s it was the norm to possess one. Ireland was usually slow to adopt European trends but it adopted surnames in the 11th and 12th centuries. Irish registrars like Matheson faced considerable difficulty with the great variation in surnames presented to them. One can only imagine. The following example is attributed to Matheson's Special Report. I laughed when I read it so am sharing the story with blog readers:

Some years ago the marriages of a brother and of a sister in the same family were solemnized in a Registrar's office. The son gave his surname as 'Faulkner' and his father's surname as 'Faulkner'. The daughter gave her surname as 'Falconer', and her father's surname as 'Falconer'. Both marriages were subsequently re-solemnized in a place of worship, and the same name orthographical differences were found to exist in the records kept by the officiating minister. I wonder if Faulkner descendants doing family history research ever figured this out.