Costello is a Norman-Irish name, like Burke, Butler, Doyle, any of the Fitz names etc. The only solid connection with Spain is that the original inhabitants of Ireland came from Spain around 10,000 years ago. As a result, genetics have found a strong link betweeen Basaque and West Coast Irish- where the darker Irish are prevelant. The Celts who invaded Ireland later brought with them the traits of blondism and red hair. The myth was that the survivors of the Armada landed in Ireland- it is false and fanciful. There are some isolated instances of Spanish families in Ireland but ther reverse was much more prevalent- "Flight of the Earls"
The most famous Irishman of "Spanish blood" was Irish President and Taoiseach Eamonn de Valera who was born in New York City to an Irish mother and Hispanic father.
My wife has her family tree from Ireland traced back to 680AD. Their are spanish names a few years after the Armada incident.
I don't doubt the veracity of your statement. 680 ad is way back-even before the Irish had surnames really. I am an Irish history buff and would be interested in the history. The whole crux of my statement is that the tendency towards black hair etc. is something that was indigenous to the Irish people for millenia before the Armada. There might have been a few stragglers from the Armada who made it to Sligo but they did not change the gene pool. My surname comes from the Irish language for Tribe and some like to fashion it the oldest Irish name. (Tuatha) De Danaan. I realize though that my family were tinker, dirt farmers.
phkrocket metioned how this thread has gone off in a total tangent. He is right but that is what I like about this site. There are a lot of people with such contrasting values and ideas that it help[s you focus your own preconceived notions.