Ryanair doesn't need a new aircraft, just a new model. The Boeing 737 for which all their pilots are trained is available in an ER version (both 737-700ER and 737-900ER).
The 737-700ER has a of 5510 miles. This will allows Stansted to Islip (3478 miles) or Oakland! (5375 miles). Even San Diego (mentioned in the announcement) is possible.
We know that planes get harder to fill the larger they get. Typically the carrier that flies to a destination the most frequently with a competitive CASM. All of the LCC efforts (Laker Airlines, People Express, etc) that were transatlantic in the past used bug metal (DC-10s or 747s) and had trouble consistently filling them up. The larger carriers forced international service to be restricted to their hubs to build the demand to fill the airplane.
Now with openskies the Low Cost Carriers can access the European markets much more easily. But all of the carriers are built on minimizing the number of different airplanes in their fleet to reduce maintainence cost. It seems a streach for a Airbus carrier like F9 to start operating a 757 or even a 330 for transcons with the rest of their fleet oriented around smaller seat counts.
The 737-700ER has more then a thousand nautical miles on any other narrowbody (5,510 to be exact - the 757 had 4,100 and the 319LR has 4,500nm). That would allow for non-stop service from ATL or even LAX to LHR. And with 1/2 to 1/3rd the seat count of a 767 or 747 it should be much easier for the LCC's to use for OpenSkies service.
With all that in mind, does this put FR, WN and FL in a great position to start transatlantic service with a plane that already fits into their fleets pretty well? Will the US-EU openskies agreement spur demand for this plane? Will the Airbus low cost carriers B6 and F9 have to look to larger jets since the A319LR doesn't can't do LGB to LHR or DEN to LHR? Would the CASM difference between a 737-700ER and larger widebodies kill the idea?