As of April 1 2012 IATA rule 302 applies to baggage allowances and fees. This is explained in this article fairly well. More can be googled.

http://www.businesstraveller.com/asi...ggage-policies


Through this rule IATA introduced the Most Significant Carrier (MSC) concept where the baggage allowances of the carrier to cross a region--typically the longest flight applied to the entire journey. They specify three regions, the Americas, Europe/Middle East/Africa and Asia Pacific as the regions.

Since Dipesh Gre Gre is crossing regions on EY, Aer Lingus will have to comply with EY's baggage policy as will Air Berlin because they are the MSC.

The exception is to flights originating in the US where the DOT defines the MSC as the marketing carrier. Under this scenario if Delta markets a flight on AF ATL/CDG/AMS with the last segment on KLM Delta's bag rules apply throughout.

However if Delta markets a flight YUL/CDG/AMS flown by AF and KLM, the Air France baggage rules apply since AF is the MSC and the flight did not originate in the US.