It sure is a rare name. I will copy the meaning I have found here and not try to simplify it in my own words, so that I may not change it somehow:
Irish: Reduced Anglicized form of Gaelic Mac Oisdealbhaigh 'son of Oisdealbhach', a personal name composed of the elements os 'deer', 'fawn' + dealbhach 'in the form of', 'resembling'.
For more information on Costello family history, go to this link:
