Yes, you can use United MileagePlus miles to book award flights on Aer Lingus, as the two airlines maintain a long-standing codeshare and frequent flyer partnership despite being in different alliances (United is in Star Alliance, while Aer Lingus is unaligned but owned by IAG). To book, you simply log into your United account, select "Book with miles," and search for your desired route, such as from the U.S. to Dublin or Shannon. Aer Lingus award seats will appear in the search results alongside United-operated flights, provided there is "Partner Award" availability. Additionally, you can earn United miles when flying on paid Aer Lingus tickets by adding your MileagePlus number to the reservation. This partnership is particularly valuable for travelers looking to avoid the high fuel surges often associated with other trans-Atlantic carriers, as United generally does not pass these fees on for Aer Lingus redemptions.