In the book Irish Families, by Edward MacLysaght, the source for the name Costello is stated to be "MacOisdealbhaigh, anglice MacCostello." I have no idea as to how to pronounce "MacOisdealbhaigh" but would appreciate any pronunciations available.

It's a little like asking how to pronounce "Wocestershire". At one time, long ago, all the consonants were said, but not today, or any time recently. The more modern spelling of your surname is Mac Coisteala, roughly mak KOSH-chuh-luh, and the old spelling would be pronounced the same today.

Music lovers who admire the harp compositions of Turlough O'Carolan are sometimes dismayed to see his first name written in the old spelling: Toirdhealbhach.

Dennis
http://www.eskimo.com/~donncha/stair3.html

For years I thought Costello was an Italian name by the look of it. It was interesting to discover some years back that it was actually Irish. So I assumed that the comedian Lou Costello was Irish-American.

Wrong again. My father and his family were neighbors of his in Paterson, New Jersey, and knew his brother (who 'was funnier but never went anywhere') and mother. Their original name was 'something like Christello' and they were, in fact, Italian.

Many people changed their names when they came to the U.S. so they would fit in more easily. In cities in the east where the Irish often dominated politics (and jobs), the new name could look Irish in addition to the more neutral English or Welsh. I once knew a Polish-American McCloskey.

Seosamh McCloskey