Following shakedown, "Irwin" departed San Diego 26 April 1944 for Hawaii, thence to Eniwetok to stage for the invasion of the Marianas. She sailed 11 June in the screen of escort carriers providing air cover for the invasion of Saipan 15 June 1944. As the Japanese Mobile Fleet was turned back in defeat from the Marianas in the Battle of the Philippine Sea 19 to 21 June, "Irwin" shot down an enemy torpedo bomber while repelling air attacks on the escort carriers.