From the EI IPO prospectus:

In 1999, Aer Lingus entered into a strategic alliance with British Airways and American Airlines. As part of the strategic alliance, Aer Lingus entered into codesharing agreements with British Airways and American Airlines, as well as agreements in relation to the co-ordination of frequent flyer programmes and access to business class lounges.

On 1 June 2000, Aer Lingus extended its strategic alliance with British Airways and American Airlines by becoming a member of the oneworld alliance, the global airline alliance formed by British Airways and American Airlines and comprising a number of international airlines.

The primary objectives of Aer Lingus’ strategic alliance with American Airlines are to ensure connectivity to US points beyond the four gateways served by Aer Lingus and to benefit from the American Airlines global sales effort for high yield passengers. Under the terms of the codesharing agreement, American Airlines may sell tickets and place its code on an Aer Lingus operated transatlantic flight when sold as part of a connecting journey from another US city (e.g., Dallas-Chicago-Dublin). Similarly, the primary objectives of the strategic relationship with British Airways are to enable Aer Lingus to compete effectively on a worldwide basis. Under the terms of the codesharing arrangement, British Airways may sell tickets and place its code on Aer Lingus operated services between Ireland and London Heathrow when sold as part of a connecting journey (e.g., Dublin — London Heathrow — Bangkok). For 2005, approximately 5.4% of Aer Lingus’ passenger revenues derived from its interline and codesharing arrangements with American Airlines
and British Airways (Source: Management accounts and internal financial and operating reporting systems).

On 30 May 2006, Aer Lingus announced its intention to exit the oneworld alliance, which is expected to become effective in April 2007. Since Aer Lingus joined oneworld in 2000, its business model has changed
fundamentally as Aer Lingus has repositioned itself as a low-cost, low-fares airline. As a result, membership of the oneworld alliance has become less relevant for Aer Lingus. However, going forward, it is part of Aer Lingus’ strategy to maintain strong bilateral relationships with its existing oneworld partners. In particular, Aer Lingus intends to maintain codesharing and interline arrangements with British Airways and American Airlines and negotiations are currently ongoing with American Airlines and British Airways to determine the shape of new arrangements which will come into effect following the exit of Aer Lingus from oneworld.

It is the intention of Aer Lingus that the new bilateral agreements will retain the key elements of its existing arrangements with both American Airlines and British Airways.
In addition, Aer Lingus has, since 1995 operated pursuant to a codesharing agreement with KLM. KLM codeshares on Aer Lingus operated flights between Dublin and Amsterdam and Cork and Amsterdam. Aer Lingus also currently has interline agreements in place with 36 carriers.