In the 1950s Pamela Jackson, an Englishwoman who lived at Tullamaine Castle near Fethard, Co Tipperary decided to leave Ireland. She had separated from her husband, Derek Jackson – a scientist and accomplished amateur jockey. The couple had moved to Ireland after the second World War. Before leaving Ireland – for Switzerland, where she lived for the next decade – Mrs Jackson called to say goodbye to the local Church of Ireland rector Canon Hazelton and gave him a farewell gift of a portrait of herself as a young woman.

The painting has been in the family ever since. But now, more than 60 years later, it has been consigned to auction at Adam's in Dublin and turns out to be a portrait of Pamela aged 18. She was, then, Pamela Mitford – one of the six famous sisters – who enthralled and sometimes scandalised British high society during the 20th century. Pamela was less well-known than Nancy (the novelist); Diana (who married, first Bryan Guinness and then Sir Oswald Mosley); Unity (who admired Hitler and shot herself in the head – but survived – when Britain declared war on Germany); Jessica (who became a communist in America); and Deborah (who married the Duke of Devonshire and became chatelaine of, among other houses, Lismore Castle in Co Waterford).

The portrait was made by the French artist Paul César Helleu (1859 – 1927) – a well-known French painter who specialised in portraits of society women – and dates from about 1925 during a visit by Pamela to Paris around the time of her 18th birthday.

It will go under the hammer in Adam's art auction in Dublin on December 2nd with an estimate of €20,000-€30,000. Pamela wasn't the only Mitford girl with an Irish association. Diana, Lady Mosley and her husband lived for a time in Clonfert Palace in Galway and then at Ileclash House in Fermoy, Co Cork. Deborah, as the Duchess, and later Dowager Duchess, of Devonshire, was chatelaine of Lismore Castle in Co Waterford.

The girls' cousin, Clementine Mitford, became Lady Beit when she married Sir Alfred Beit and moved to Russborough House in Co Wicklow. After living in Switzerland for a decade, Pamela Jackson returned to England and lived in Gloucestershire. She died in 1994. Her former Tipperary home at Tullamaine was sold by her estranged husband in the late 1950s and is today a stud farm.