There are three options:
1) Rail:
Two through train departures daily from Porto Campanha station to Vigo in Spain at
07:55 arriving 12:15
and
17:55 arriving 22:37
Connecting Through trains to A Coruna are:
12:45 arriving 15:50
05:50 arriving 08:50 (next day)
For sources see:
www.cp.pt
www.renfe.es
2) Hire a car but there will be a hefty drop of charge for one way.
3) Coach
There is a direct service operated by Internorte - see:
www.internorte.pt
On week days there is a service leaving Porto Sá Carneiro Airport at
08:15 arriving 12:45
14:45 arriving 19:45
On Saturday and Sunday there is only one service at
14:45 arriving 19:45