Is there a ferry to the Cíes Islands from A Coruña?
No.
There is no line to the Cíes Islands from A Coruña. A Coruña is in a different province and on a different coast: the Cíes belong to the municipality of Vigo, at the mouth of its ría, well into southern Galicia. There is no boat from A Coruña, so the trip has two legs — getting down to a Rías Baixas port and crossing from there — and it is worth planning it as a full day out, not a half-morning escape.
The nearest port that works
From A Coruña, the practical move is to head to Vigo and board there: €24 per ticket and 10 departures a day in high season.