American Express tells me that their Membership Rewards program will be adding Aer Lingus as a 1:1 points transfer “at the end of this month.” That will mean American Express Membership Rewards points transfer to,

• Airlines: Aeromexico, Aeroplan, ANA, Air France KLM, Alitalia, Emirates, Cathay Pacific, British Airways, Delta, El Al, Etihad, Hawaiian, Iberia, JetBlue, Singapore Airlines, Virgin Atlantic
  
• Hotels: Choice, Hilton, Marriott (later this month)

Of course American Express Membership Rewards transfers to British Airways and Iberia already, and all three airlines are owned by the same company, IAG, and all three use Avios as a currency. Provided you’ve had your accounts open at least 90 days and each has had activity of some kind you can transfer between those accounts. (Your accounts cannot be linked as part of a family account, and personal details must match exactly.)

British Airways First Class

So the ability to transfer to Aer Lingus directly doesn’t add very much, it does add the ability to transfer without going through an additional step. However since British Airways points already redeem for Aer Lingus even having points in the Aer Lingus program offers limited value. It’s mostly duplication with a more limited set of partner redemption options.

Chase offers the British Airways Visa Signature Card, and since the spring Iberia, and Aer Lingus credit cards.

Points from the Chase Sapphire Preferred Card and Chase Ink Business Preferred Card, among others, transfer to all three programs as well.

Initially when Chase re-upped their British Airways deal in 2015, American Express reduced the value of points transfers to British Airways and Iberia by 20%. However those have back at 1:1 since last summer.