I don't see EI opening up a direct route from LHR to the US; they need to focus on what they do can do best and that means flying Ireland-US routes, and - ultimately - a route from whatever EU city they pick as their next base. The intense competition on LHR-US flights from US and UK airlines alone will mean competition is intense.
The one thing that kind of worries me know is that there are increasing concerns about an economic downturn, due to concerns over US companies pulling out of Ireland and more recently, concerns about the US mortgage market. It would be very frustrating for EI to get this exciting new access, only to have these superb new opportunities undermined by a US or global economic downturn.
That said, people will still need to fly and Ireland may still have an advantage, in that it will be far better and more conveniently accessible to/from the US (and indeed Europe) than ever before, which will hopefully result in Ireland being perceived as a very convenient "entrepot" for US (and ultimately Canadian) access to Europe. Get a decent hub up and running at DUB and an efficient, lean t/a operation and EI should be well able to withstand the economic problems and challenges on the horizon.