Since amhach is feminine should fada not be lenited?
No, as it's a predicate ie 'something is something'. But if you said 'I have a long neck' ie attributive, then it would be 'Tha amhach fhada agam.' Probably haven't explained that well but it's the best I can do!
Thanks junkming1, that has explained a lot. I think I've seen this before but thought I was looking at a masculine noun. Ceud taing, Anndra