IAG chief executive Willie Walsh believes Aer Lingus would "probably struggle" to grow its transatlantic market "without a strong partner in the US".
Under the merger plans set out by Walsh's group, Aer Lingus would join the Oneworld alliance and the joint-venture business IAG operates with Finnair and American Airlines on transatlantic routes.
Walsh says also that there could be "opportunities" for Aer Lingus to grow its current transatlantic network within an expanded IAG. He gives the example of Shannon airport, where there was "very strong demand from the US into the west of Ireland" and "strong natural traffic flow, particularly in the summer".
Aer Lingus's website indicates that it codeshares with United Airlines and has an interlining arrangement with another US carrier, JetBlue Airways.
Information on Aer Lingus's existing arrangements with US airlines has been updated in this story