How did this come to be? Did the Irish own a lot of slaves?
IIRC, some people freely chose new names upon emancipation, presumably names they admired. Perhaps that is one source.
Many Southern plantation owners were of Irish descent.
This relates to Scottish, not Irish, but my family adopted a couple of orphaned ex-slave kids right after the civil war, and there’s a few black McAllisters and Alexanders out there because of it. I know we weren’t the only family to do that.
For example, Scarlett O’Hara’s father.
Many were of Scotch-Irish descent in particular.
Irish-born people in the 1860 U.S. census:
Virginia: 17,065
Tennessee: 12,368
Georgia: 6,808
Alabama: 5,682
South Carolina: 5,079
Texas: 3,486
Florida: 939