Couple of things to consider - firstly, you are at the mercy of the check in agent - you could try telling them that you will have liquids meeting you in London that you will need to put into your checked bags? Secondly, know that your ticket means they will get you from Thailand to the US - if there were delays, equipment changes etc. they could put you on a different routing to the US (i.e. via somewhere other than London, or direct to the US on their own or another airline's flight). Something similar happened to a friend last month, they were due to fly A-C via B. But their A-B flight was delayed so much that they would miss their B-C connection, so the airline routed them from A-D-C instead. It is unlikely, but you might want to make sure you Ireland part is fully refundable just in case.