From the FAQ:
Q - I have already used my unique presale access code to buy tickets for U2ieTour shows earlier in 2015, can I use it again in presales for the Belfast/Dublin shows ?
A - If you've already bought two tickets in earlier pre-sales, your presale ticketing code is no longer valid. If you only bought one ticket in an earlier presale, your code will be valid to buy one more ticket in the Belfast/ Dublin presale.
Yes, but if you have NOT used your presale access code before, can you buy a single ticket from 2 different shows in Dublin? So you put 1 ticket in shopping basket an