The Hon. Mary Molesworth, daughter of Field Marshal Richard Molesworth, 3rd Viscount Molesworth and Jane Lucas, married Robert Rochfort, 1st Earl of Belvedere, on 7 August 1736, when he was 28 and she 16. Despite his fairly rapid rise at court and in politics, Rochfort is probably most well remembered for his treatment of Mary. Around 1743, he heard rumours that Mary had been unfaithful to him with his brother, Arthur. As punishment, Robert had her locked up in the family house at Gaulstown, alone apart from her servants, for the rest of his life (thirty-one years). After twelve years of this captivity, she attempted to escape but was caught and subjected to even harsher treatment. When she was finally released by order of her son after his father's death, she apparently took to wandering the house and talking to portraits as if they were real people. Her voice had assumed a peculiar quality (like a shrill whisper) and she was obviously profoundly damaged by her experience. After Robert's death, her sons immediately released her. To aid her recovery she accompanied her son, George, 2nd Earl of Belvedere, on his honeymoon to Italy. On their way via France she felt unable to cope with travelling and spent some time in a nunnery while her son and his new wife carried on to Italy. This gave rise to the rumour that she joined the nunnery and died a hermit there. On their return she spent a year in London in apartments in Kensington Palace. Once her story became more commonly known she struggled in society and returned to spend her last years in Ireland. She spent much time at her son's house (Belvedere House) in Great Denmark Street, Dublin. She then spent her remaining years with her daughter, Jane, Countess of Lanesborough.
Saunders's News-Letter - Saturday 27 August 1785 , p.2 reported her death "At the Black Rock, the Right Hon, the Countess Dowager of Belvedere."
She was not the only one to suffer though, as Robert took his brother Arthur to court for criminal conversation and was awarded the huge sum of £2,000 in damages. Arthur, unable to pay, fled the country, but on his return to Ireland, he was thrown into Browne's Castle (used as Dublin's debtors' prison at that time, and famous for being "the worst prison in the country") where he died in 1774, a few months before his brother Robert.
For a fuller account see E. Littel, Littel's Living Age, vol XII (1847), p.121-25 (Public domain).