My wife asked me “How come Costello is an Irish name? It doesn’t sound Irish.” I had to agree that it didn’t, so I checked my go-to book for surnames, Rybakin’s Словарь английских фамилий = A Dictionary of English Surnames (Moscow, 1986), where I found that it represented an Irish MacOisdealbhaigh, pronounced something vaguely like mc-ISH-dalwa, and you can sort of see how that could wind up as Costello, especially after a few pints. She raised an eyebrow when I reported this but didn’t object. Then I made the fatal decision to check Wikipedia, which said “Oistealb or Osdealv was the Gaelic rendering of Jocelyn,” referring me to this webpage:
The origin of the surname Costello provides a perfect illustration of the way the native Irish absorbed the invading Normans. Soon after the invasion, the de Angulo family, also known as “Nangle” settled in Connacht, where they rapidly became powerful. After only three generations, they had begun to give themselves a surname formed in the Irish manner, with the clan taking Jocelyn de Angulo as their eponymous forebear. Jocelyn was rendered Goisdealbh in Irish, and the surname adopted Mac Goisdealbhaigh, later given the phonetic English equivalent “Costello”.
This time my wife said “I’m sorry, that makes no sense,” and I find it hard to disagree. How on earth do you get Goisdealbh out of Jocelyn? But I’m not the explainer here, just the bearer of odd news. I report, you decide.
(Wikipedia also says: “Although it is not of Italian origin, the name Costello has a misleading Italian appearance. It occasionally has been adopted as a pseudonym or stage name by famous people of Italian descent [...], which creates further confusion about the origin of this Norman-French surname.” It sure does!)