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.