I booked with AA before to Chicago and Aer lingus were used, I was told at the time that they shared aircraft on the route. Al are supposed to be leaving the Oneworld alliance early this year, but I thought that they would to still codeshare with AA, the flights appear different alright.
I don't believe that their is huge competition on price to the States though. This time last year I booked tickets to Florida during December that cost €460 return with Delta, Aer lingus later put a fuel surcharge on their routes to the US of €80 return, and low and behold Delta are charging a bit over €80 extra this year on the route, and most routes to the big hubs in the US have risen in price across the board for the other airlines by about the same:mad: .