I would imagine, though happy to be wrong, this would be ADD-(DUB)-LAX with anyone boarding in Ireland as an added bonus. Remember Kuwait fly KWI-LHR-JFK though don't register much in the UK-US market. Beyond a point in a press release and a dot on the map, I would be surprised if it added much to the Ireland US point to point market.
AI used to do BOM-LHR-JFK with full traffic rights from the 1960s until a few years ago (and unlike KW, booze is served).
Though it's hard to believe now, AI was a top-notch carrier back in the day. It was a popular and soughtafter carrier from the start as it had a unique selling point of being different from the run of the mill TATL carriers at a time when fares were all the same and set by IATA.
Regretably AI later became crap and the advantage was squandered. Traffic levels dropped off and the LHR-JFK section of the route was abandoned.
Much depends on how well ET market and promote the route, whether they can offer competitive fares and attract premium pax, and whether they can grab a share of the cargo business. Failing that, pre-clearance will have to be the factor that attracts sufficient ADD-LAX pax.