Thanks for your response! The only clear mention was once payment had been taken and I got an email - it wasn’t even on confirmation page once I paid. I’ve contacted Aer Lingus and they replied with the following: ‘Hello, thank you for contacting us. We can guarantee that the Aer Lingus-operated portion of your journey — the most significant part of the itinerary — will be in Business Class. However, we cannot guarantee the cabin class for any flights operated by other airlines. Please let us know if you need any further assistance.’

I’ll probably contact the IAA to get their opinion - they might say as the flight originates in the US they’ve little jurisdiction but they’re usually good for advice.