This is a much travelled and very complicated surname with several origin splits. In its simplest form, it is locational, and probably derives from the Staffordshire village of Cosely, a name which is of Olde English derivation, and translates as "the charcoal burners enclosure". It is probable that most of the Irish nameholders, particularly if recorded in Ulster, are from this source. However, outside of Northern Ireland the nameholders descend from the Gaelic "MacGoisdelbh", which is today mainly Anglicized as (Mac)Costello. The curiosity here is that the origin is the Norman "de Anglos", and was the first example of a Norman family taking an Irish name, probably in the 13th Century. The similarity of the sound between Costello and Costley accounts for its use, a link form being Costelly, one Betty Costelly being recorded in the Famine Records on April 29th 1847, when she embarked for New York. Other recordings include: Lawrence Cosley, of Dublin, on December 26th 1710; Thomas Casely, of Downpatrick, County Down, on April 22nd 1773; and John Costley, a witness at Moira, County Down, on February 28th 1790. The first recorded spelling of the family name is shown to be that of Margaret Costlye, which was dated October 17th 1603, marriage to Thomas Howe, at St. Botolph's Church, London, during the reign of King James 1 of England and V1 of Scotland, 1603 - 1625. Surnames became necessary when governments introduced personal taxation. In England this was known as Poll Tax. Throughout the centuries, surnames in every country have continued to "develop" often leading to astonishing variants of the original spelling.