Can I use preclearance for a flight Dublin to USA that connects in Heathrow?

The question is base on a misconception of US preclearance. For practically every country, once you arrive on their soil from outside^[1] you are subject to passport controls and customs inspection. This means that:

• you can only arrive at ‘international airports’, i.e. those that have these facilities
• if there are a number of flights arriving at the same time, customs and immigration may be very congested.

Preclearance was introduced to combat these points, originally by a request of American Airlines who presumably wanted to operate flights from Toronto to ‘non-international’ US airports (where immigration and customs cannot be performed on arrival due to lacking facilities). Its concept is to ‘outsource’ all immigration and customs checks to foreign soil, before a plane’s departure. This renders an international arrival something that seems like a domestic arrival from the arrival airport’s point of view. And it makes sense from a time-economic point of view: passengers typically arrive at the departure airport many hours before their plane leaves, so why not use that spare time so that they can make quicker connections on arrival?

The big downside to preclearance is that it requires those facilities at the departure airport and it also requires a bilateral agreement between the US and whichever country the airport is in to allow for US CBP to perform their duties in foreign countries.^[2]

Your suggested itinerary, however, completely defeats the idea of preclearance. Say you are in a plane with 200 other passengers on your way to Heathrow from Dublin. You might be travelling on to the US and maybe ten others, too. That leaves 190 passengers in that plane not going to the states that don’t need to (and don’t want to, and must not) go through preclearance — they will not enter the US.

You could make arrangements to have somebody else smuggle some customs-relevant item onto your Dublin–Heathrow plane that you then intend to get past the US customs as you already went through preclearance. That would create a massive loophole. Therefore, preclearance is only possible for those flights that depart for the US without further ado.

And what about your Heathrow–US flight? Well, Heathrow airport does not have preclearance facilities.^[3] So you cannot go through preclearance there.

[1]: Sometimes, outside does not only mean outside the country but outside of a greater set of countries. E.g. Arriving in Germany from the Schengen area or in the UK from the Repulic of Ireland (as part of the Common Travel Area) is basically treated as domestic travel.

[2]: Typically, the US CBP has less possibilities when operating on foreign soil. For example, they do not have the power to arrest travellers outside of the US, and they cannot force people to allow searches. They can, however, prevent boarding.

[3]: Plans exist to extend preclearance to a number of additional airports; Heathrow is amoung them. (Homeland Security press release)