The site for tickets sales was blueticket.pt. Now there is not even a mention of U2 on there as they sold out so fast. We managed to get some for the first show from them through the u2.com presale, for the second show we just could not get through to their server (and were trying desperately from a McDonald's in the middle of Germany where we paid 8 Euros for internet access !) & finally we have ended up paying approx. double face from Viagogo - the tickets seem to be like hen's teeth and we didn't want to risk waiting for a last minute ticket drop/ outside stadium dodgy deal!
Good luck. Got high hopes for the Coimbra shows - nice smaller stadium & good Portuguese fans.