Coadjutor bishop
On 25 April 1942, he was appointed Coadjutor bishop of Meath and Titular bishop of "Binda". D'Alton received his episcopal consecration on the following 29 June from Cardinal Joseph MacRory, with Bishops Edward Mulhern and William MacNeely serving as co-consecrators, in the chapel of St. Patrick's College. He succeeded Thomas Mulvany as Bishop of Meath on 16 June 1943.