The theory is this:

The city is founded in 9th century AD by Arabs (althoug some say it was already founded in the Byzantine empire) and got the name Qurliyun. When the Italians got the city they named it first Cuor Di Leone and after that just Corleone. Interesting fact: Some want to change the name back to Cuor Di Leone, just for the bad looks Corleone gives them (Godfather movies, Toto Riina etc)