Costello is an Irish and Italian surname. There is no credible evidence that the Costello surname of Ireland is connected to the Spanish surname Castillo or that the Costello family originated with shipwrecked Spanish sailors during the failed Spanish Armada invasion of Britain.
The surname has been borne by a notable Irish family who claimed descent from Hostilo (or Hostilio) d'Angulo, an Anglo-Norman knight. Hostilio d'Angulo accompanied his father, Gilbert d'Angulo and brother Jocelyn d'Angulo to Ireland in 1172 under the leadership of the Earl of Pembroke Richard de Clare, known as Strongbow.
The invasion was of course ultimately under the command of Anglo-Norman king, Henry ll. Pope Adrian IV, the first English pope, in one of his earliest acts, had already issued a Papal Bull in 1155, giving Henry authority to invade Ireland to bring the Irish Church into conformity with Roman practice.
It is reported that the surname d'Angulo derives from Angle, Pembrokeshire where the d'Angulo's resided prior to the invasion.The family first appears on record in Ireland in 1193, when the Annals of the Four Masters state: Inis Clothrann do orgain la macaibh Oisdealb, & la macaibh Conchobhair Maonmaighe. (Inishcloghbran was plundered by the sons of Osdealv, and the sons of Conor Moinmoy.)[1] Oistealb or Osdealv was the Gaelic rendering of Hostilo.
The Irish name Oisdealbh is sometimes erroneously attributed to a Gaelic translation of Jocelyn, Jocelyn d'Angulo's given name. In fact, MacOisdealbhaigh is the Gaelic translation of his brother Hostilo (or Hostilio) d'Angulo's first name.
Oisdealbh is a much more obvious translation of Hostilo. It is Hostilo's descendants who would bear the surname Mac Oisdealbhaigh (son of Oisdealbh or Hostilo).
The surname would later be anglicized to Costello, Costelloe, and Costellow. Gilbert d'Angulo is also sometimes listed as the son of Jocelyn d'Angulo in error. Gilbert d'Angulo is the father of both Jocelyn and Hostilo.
In Italy, the name originated in the Campania region and in Sicily but is now more prevalent in the northern regions of Italy. Its origins as a surname are from the medieval Latin name "Costellus". It is related to Costa, a more common Italian surname, as its diminutive.
The title originated with the appointment of Michele (Costello) as the Consul of Belluno in northeastern Italy in 1378.
It occasionally has been adopted as a pseudonym or stage name by famous people of Italian descent, including Al Costello (né Giacomo Costa), Frank Costello (né Francesco Castiglia), and Lou Costello (né Louis Francis Cristillo).