Complex sentence: The island was named Wells Island but renamed by Captain William Fitzwilliam Owen to the current name, but there is no indication whom it was named for, but likely for Richard Wellesley, 1st Marquess Wellesley.
Split into simpler sentences: The island was named Wells Island but renamed by Captain William Fitzwilliam Owen to the current name. There is no indication whom it was named for, but likely for Richard Wellesley, 1st Marquess Wellesley.