The name is derived from the Irish Gaelic sept Ó Cléirigh, meaning "clerk".
The English version is of Anglo-Saxon origin and was used in the Middle Ages for the name of a scribe or secretary.
The word "clerc", which came from the pre-7th century Old English "Cler(e)c" (meaning priest), originally denoted a member of a religious order, but later became widespread.
In the Middle Ages, virtually the only people who could read and write were members of religious orders, linking the word with literacy.
Thus the surname became a popular term for a literate man, particularly for the professional secretary and the scholar.