Leaving/arriving layover airports
 

I'm looking at flights from NYC to Heathrow, quite expensive at the moment but I have found cheaper flights to Dublin, using Heathrow as a layover. My question is, as I am not checking in luggage, if I buy this ticket can I just use Heathrow as my arrival and departure airport?

I believe that, if you don't take the Heathrow to Dublin leg, your return flights will be cancelled.

That is correct. You'll lose your flight home.

Then Book 2 one way tickets. I know AA does that. So do most other airlines, without a surcharge.

Except if you're suggesting booking a one-way return DUB-LHR-JFK, the OP cannot board in London.

<i>Then Book 2 one way tickets. I know AA does that. So do most other airlines, without a surcharge.</i>

As soon as OP is a no-show for one leg of their trip, all subsequent legs are cancelled - so for a one way DUB-LON-NYC flight the LON-NYC leg would be cancelled as soon as they don't show for the DUB-LON leg