Investors, including the Irish government and rival airline Ryanair, now have to decide if the bid of 2.50 euros per share plus a cash dividend of 5 cents a share is enough to win them over. The Irish airline's board said it was considering the offer.
The Irish government is likely to come under pressure over the offer. The air traffic provided by Aer Lingus is a key driver of the Irish economy. Opposition party Fiánna Fail, and the Labour Party, who are a junior party in the coalition government, have already expressed concerns about the bid.
Aer Lingus floated at 2.20 euros in 2006. However, the Irish airline has consistently traded at below its IPO price since March 2008. Its margins are lower than IAG's, and Ryanair has seriously dented its performance on short-haul flights. Aer Lingus's pension scheme has also been the focus of recent controversy in Ireland, after a deal between the company and the trade unions was signed to cut its deficit.
The European competition authorities, who have previously scuppered Ryanair's bid for Aer Lingus, will be looking closely at this offer. The main area of focus will be Heathrow, where IAG (via British Airways), controls around 51 percent of slots – a level that would potentially rise to around 55 percent (according to Nomura analysts) if it bought Aer Lingus. The Dublin-Heathrow route is one possible area of focus.
- By CNBC's Catherine Boyle