But what most of Paris's bon viveurs didn't know about Marie-Louise Morphi was that she was Irish: the daughter of a cobbler and an ambitious Irish mammy who thrust her five daughters into the Parisian demi-monde to make their fortune.

They succeeded quite spectacularly . . . and, in the case of Louisa, almost brought down the monarchy. When she gave birth to her first son by the king in 1754, the British ambassador wrote:

"Whether this will occasion a revolution with respect to the reigning favourite will be known in a little while".

It didn't, but Louisa remained close to the king for years longer than any of his other mistresses, and was imprisoned for two years after the revolution on account of it.

Her father had arrived in France from Ireland to serve with an Irish regiment of the French army. He later settled in Rouen as a cobbler, but his wife, Margaret Hickey-Murphy, had loftier ambitions. She arrived in Paris with her daughters in 1741, and quickly saw her eldest, Marguerite, 15, installed as mistress to a close friend of Casanova.

The younger girls sought positions as models at the Academy of Painting, where Louisa became a favourite.

When Mme de Pompadour, the declared mistress of the king, commissioned a painting of the Holy Family for the Queen's Oratory, she insisted Louisa pose as the Madonna and it was through that painting that Louisa came to the king's attention.

Their affair lasted for three years, during which time Louisa gave birth to two children, both of whom were taken from her and placed with foster parents.

After the affair ended, Louis arranged for Louisa to be respectably married and she produced a third child, a boy. It was said, however, that the boy favoured the king, whom Louisa still occasionally saw.

Tantalisingly, a story prevails that the boy, who later became an officer in the revolutionary army, was the man who ordered the drum roll to drown out Louis XVI's attempts at a final speech under the shadow of the guillotine. If true, it would have cast Louisa O'Murphy's son as a key player in the execution of his own half-brother.

After the revolution, Louisa remarried and continued to live in Paris until her death at the age of 79 around 1814/1815.

'Little Morphi' had lived long enough to see the monarchy she had almost toppled reinstated.