Cian O'Connor's grandparents, Dr Karl and Mrs Doreen Mullen, watched the winning round from their home in Kilcullen, Co Kildare, in front of RTÉ cameras.
Speaking in the moments after the winning round, they said they were "thrilled and delighted" with the result.
Dr Mullen, a former Ireland rugby international and Triple Crown winner, said: "It is the most wonderful prize in the world and it has been won by a young Cian O'Connor. It is a stunning achievement, just fantastic."
Cian's grandmother, Mrs Doreen Mullen, said: "I'm almost speechless. I can hardly speak. It's simply a feeling of utter joy - I'm so, so proud. I had no idea he would win gold. Beforehand if he had won the bronze, I would have been delighted."
Tributes were paid by the Tánaiste, Ms Harney, the Minister for Sport, Mr O'Donoghue, and Mr John Foley of Waterford Crystal, sponsors of the winning horse.
Cian O'Connor's godfather, Sir Anthony O'Reilly, said he was "delighted by Cian's triumph both for himself and his family...It is a great day for Cian, for Ireland and for everyone involved."