Aer Lingus has become the first carrier to sign a partnership with Heathrow Express to allow online bookings of the premium train as part of flight itineraries.
More carriers are due to sign up for the option, which incorporates the purchase into the flight booking and produces a separate e-ticket.
Fares are the normal £34 return standard class (£17 single) or £52 Business First return (£26 one way), the same as if purchased at the station on the day.
Aer Lingus was selected as the first partner airline because of its existing range of online ancillary services and large number of business travellers from Ireland.
The carrier operates up to 12 flights a day between Dublin and Heathrow, four a day from Cork and three a day between Belfast and Shannon and Heathrow. Tickets for the 15-minute train journey are not sold onboard its flights.
A Heathrow Express spokeswoman said negotiations with other carriers were underway, with names to be announced “in the next couple of months”.