It appears that as part of Ryanair's commitments to secure regulatory approval to acquire Aer Lingus, BA will take over Aer Lingus's services to Ireland for a period of at least three years and in doing so acquire the right to purchase the slots belonging to Aer Lingus.
There's no certainty that the European Commission will clear Ryanair's takeover but if it did and BA acquired Aer Lingus's slots (currently 3.3% of LHR slots), it could take BA's share of LHR slots near to 56% (and cause more terminal planning problems!).
http://www.ft.com/cms/s/0/28317d80-4...#axzz2F3kADQ1J
The UK flag carrier, the largest airline at Heathrow, has struck a deal with Ryanair to purchase more than 85 per cent of Aer Lingus’s slots at the airport, which are currently used to provide services to Dublin, Shannon and Cork, said three people familiar with the agreement between Ryanair and British Airways.
[...]
Ryanair is trying to secure regulatory approval for its third takeover bid for the Irish flag carrier.
Last month, the European Commission objected to Ryanair’s bid on competition grounds. The commission prohibited Ryanair’s first bid for Aer Lingus in 2007, and Brussels has never cleared a merger that it previously rejected.
British Airways is offering to take responsibility for many of Aer Lingus’s services out of Heathrow for at least three years.
With British Airways operating these services, Ryanair is privately saying that a combined Ryanair-Aer Lingus would not be dominant on those routes, said people familiar with the Irish budget carrier’s stance.
However, British Airways would run these services in place of Aer Lingus for between three and five years – after that it would have the right to buy the Irish flag carrier’s Heathrow slots and reallocate them to different destinations, such as New York.