Ballynatray Demesne overlooking the Blackwater estuary above Youghal is a unique combination of natural and designed landscape. Its new owners, Serge and Henriette Boissevain, have renovated the landscape, planted new woods and completely restored the house which dates from 1795. Here Patrick Bowe outlines the history of the estate and of the Smyth family which created it while Deirdre Conroy gives the architectural detail of the big house