I don't really think that this new idea will work, and whilst I know that people do say this everytime someone comes up with a new idea, a number of airlines "promising something new & different" vanish from the air space as quickly as they arrived with the whirl-wind of publicity.
High quality service with low fares has been tried before in the UK & Ireland - Debonair. It failed miserably because of a bad cost base which didn't allow it to keep up with EasyJet & Ryanair.
Ryanair already flies between DUB & AGP, and serves from many airports Murcia (about 1 hour from Alicante) do who knows if they will launch that route from Dublin as well.
Either way, the company is going to come under much competition, with a product (first class means less seats = less seats to spread the cost of the flight across allowing lower fares) and aircraft type (the 757 can not do turn arounds under 50 minutes - more time on the ground = fewer rotations = higher costs) that are flawed.
Good luck, but JetGreen (doesn't it seem like projectile vomiting a jet green?) seems to have too many flaws.