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The "Costello" spelling could have originated in Italian, but it was applied to Irish named MacOisdealbhaigh.

Thank you for the unintentionally comic line of the day.

I'm sure your Irish must be right, but their spelling is really something.

[Albion Wilde]

"Costello = MacOisdealbhaigh."
Thank you for the unintentionally comic line of the day. I'm sure your Irish must be right, but their spelling is really something.

Those Irish Gaelic spellings really are complicated, aren't they? Here is a comprehensive list of Irish surnames in the original Gaelic.