It might be due to the character used for the apostrophe. I did a search for O’Carroll, and they are both in this list. Try starting the merge from O’Carroll profile, then find the OCarroll profile, and then initiate the merge.
Depending on the character set your computer is using, the apostrophe could be straight up and down, or “curly.” If it is the later, WikiTree could be interpreting it as a different character, thus the error.
I have noticed problems like this when I copy text from my word processor into my desktop genealogy program.