What is "cessair"

Cessair or Cesair (spelt Ceasair in modern Irish; anglicized Kesair) is a character from the Lebor Gabála Érenn, a medieval Christian pseudo-history of Ireland. According to the Lebor Gabála, she was the leader of the first inhabitants of Ireland, before the Biblical Flood. The tale may be an attempt to Christianize an earlier pagan myth, but may alternatively be the product of post-conversion pseudohistory.