Originally Ua'h-Anluain, O'Hanlon and Hanlon are the obvious variants of the name, but there are many more: Handlon, Handlan, Hanlan, Hanlen, Hanlin, Hanlyn.
Some cases of Hanly, Hanley, Handley are also a variant of Hanlon (though these are usually derrived from Ua'h-Ainlighe, an ancient Roscommon clan).