What a terribly demanding question. I've photographed much of Ireland's coast from Bantry round to Strangford Lough and its hard to think of an area I don't enjoy and I'd love to return to much of it.
I suppose though that if really cornered I'd pick the Connemara coast from east of Glinsk round to Clifden as the best of the best. Honorable mentions though to the northern part of Inishowen; northwest Donegal round Dunfanaghy; the coast just north of Sligo round Streedagh; and the Co.Mayo coast round Westport and Achill Island. Then of course there's the Beara/Sheeps Head and Mizen Head areas and the illogical fact that some of the best photography I've done was on the beach at Rossknowlagh right in front of the hotel with caravan parks and holiday homes all round me. I feel guilty every year I don't return.