it works very well i have used it twice, you simply need to have the phone open on the boarding page and they have a devide at the gate that scans the screen and lets you in. only works in dublin and some other airports
I guess it's similar to the British Airways one where your boarding card barcode ends up on your phone screen and they just scan your phone screen at security / at the gate .. it's very handy .. I have used mobile phone boarding cards for non Aer Lingus flights at Dublin before ( BMI used to have them before their demise) and the new security barriers they have there work fine with phone screens.
Hopefully they've built the app so you don't always have to have a data connection .. the BA one is reasonably smart in that you download the boarding card to your phone and it stores it so you don't have to be online when you're at the airport - saves the data roaming charges as you can download the boarding card on the hotel wifi and then be offline at the airport.
Friend of mine used it the other day.
Battery died in his phone just before he went through.
He had to go back to the desk to get them to print his boarding pass