When the glacier that covered Ireland retreated at the end of the Quaternary, the sea came in and filled the resulting valley floor. This formed an inlet of water that is steep, narrow and deep. To be a true fjord, the water within needs to be deeper than the sea it drains in to. This is true for Killary Fjord where the water is -40m behind the terminal moraine of the fjord and -20m in Killary Bay, the sea in which it drains in to.