The surname Clark is derived from cleric, clerk, or scholar - one who can read and write. It is from the Latin clericus, meaning scribe or secretary. It is also from the Gaelic "Mac a' Chlerich/Cleireach"; son of the cleric or, sometimes, clerk. During the Middle Ages, the common pronunciation of -er was -ar, so the man who sold items was the marchant, and the man who kept the books was the clark. At the time, the primary members of the literate class were the clergy, which in minor orders were allow to marry and have families. The term clerk (clark) eventually came to designate any literate man.
The Cleary / O'Clery surname, one of the oldest surnames in Ireland, is often anglicized to Clarke or Clark.