I don't know about Heathrow, but the USA doesn't account for that many trips to Amsterdam.
The largest group is travellers going to destinations in the low countries who are travelling on by surface transport. Next is European destinations not served directly from Cork or not served at the right frequency. After that comes eastbound long-haul (Manila is particularly popular because there are no direct flights from Heathrow). Westbound long-haul would then come after this. It could take some passengers off the Amsterdam flight, but not a massive number.