I am just not sold on the idea of long haul low cost. Norwegian had some headline grabbing fares, but I have not found them any cheaper to NYC from London than you can get on other airlines. It's hard to make price much lower on long haul, as you have much the same fixed costs on fewer segments through which to realise operational efficiencies. Moreover, the customer perception on price has to be significant for long haul. If I had a choice of United or Norwegian to NYC, then the difference in price has to be significant to make me bite- I know I could just use my own iPad for entertainment and spend £15 on a meal and drinks, but my perception is that I'd need to be saving close to £100 return to make it a valid choice. That is where I believe the model will find it difficult.
I think the one area where low costs could make entry is finding new markets to a few long haul destinations (NYC, Florida, etc.) from secondary EU airports- Eindhoven, Basel, Hamburg etc., which have been overlooked. This is, in essence, what Norwegian have done from Gatwick (i.e. getting Surrey traffic who don't want to travel to Heathrow), Bergen etc. The low costs have developed expertise in building small bases with no feed traffic, getting to understand how to scale an operation and market effectively. I think this is the way Ryanair could make a long haul entry. I just don't believe there is that much money in it.