Don't get me wrong, I really hope that Cathay would fly A350 to East Coast too.
Aer Lingus document shows that the A359 only has a range of 7000mi too
All aircraft are limited in range as you start adding payload. The 77W can carry more weight over short-medium distances, but at ultra long ranges the A350 becomes better.
Aer Lingus will have modelled the 7000nm range on carrying roughly 40,000kg of payload. At that range the 77W can carry 10t more but will use more fuel doing it. At extreme ranges (8000nm+) the A350 will outfly and outcarry the 77W. It can fly almost 10,000nm with no payload, well over 1,000nm more than the 777.