I've got an idea in my head.
One of Napoleonic France's invasions of Ireland actually succeeds in going onshore (1808-9ish). Thousands of French soldiers under General Kilmaine manage to land in Ireland by sneaking around the Royal Navy This was attempted several times in OTL but bad luck and bad sailing prevented this more than the Royal Navy. They couldn't blockade everywhere.
The Irish Catholics revolt en masse ( along with some Irish Protestant patriots) and, with about 10,000-20,000 French soldiers, manage to overrun the country's small British Regulars garrison and the Protestant Establishment militia (Wolfe Tone and other Protestant Patriots did not lead this, this was a religious rebellion).
Seeing that he cannot dominate Ireland like he does neighboring German states due to the presence of the British Navy, Napoleon decides he must cooperate and treat Ireland as an ally (a novel concept for the Emperor). He needs majority Irish support or the British will just walk back in.
Ireland wants a Catholic King. After rejecting suggestions that they accept a Bonaparte on the throne, the new Irish Catholic Parliament demands the return of the legitimate Jacobite heir.
However, Charles Emmanuel of Sardinia-Piedmont was a worn out man with no heirs and effectively deserted his Kingdom, leaving it to his younger brother, Victor Emmanuel, whom was a virulent enemy of the Bonapartes and could not be trusted.
This left Victor Emmanuel's daughter as the next in line of the Jacobite succession. As a woman, she was not going to inherit her father's Sardinian Kingdom anyway due to Salic Law. After the war, Sardinia-Piedmont would eventually go to his younger brother, the Reactionary Charles Felix.
However, Salic Law did not apply to Ireland (or Britain).
With Piedmont under occupation, it was easy to talk Victor Emmanuel into releasing his daughter to take over Ireland. In return for his daughter's good behavior as Queen of Ireland, Victor is granted Modena (ironically, the Duchy of OTL Maria Beatrice's future husband) and some other Italian territories in exchange for giving up the Piedmont for formal French annexation.
This also destroys the British-Sardinian alliance.
Maria Beatrice, 18 years old, is sent to Ireland and greeted rapturously.
Any ideas how this goes?
French(-allied) occupation of Ireland during the Napoleonic Wars would be a horrific blow to Britain. French piracy on British trade would flourish on the sea lanes with so many delightful harbors in Ireland. Britain's resources would diminish, limiting their ability to subsidize continental allies, to help defend Portugal and reconquer Spain during the Penisular war, fighting the war of 1812 with America, etc. Ireland provided a great deal of victuals for a Britain dependent imports, many exportable goods exchanged for necessary hard currency, and, bearing a large portion of the United Kingdom of Great Britain and Ireland's population, (Catholics were allowed serve in the Army since 1777ish) tens of thousands of soldiers and sailors.
Supporting Ireland (and wounding Britain) would drain enough resources from Napoleon that he might not have the resources to make one of his fatal mistakes : 1. turning on his ally in the attempted conquest of Spain or 2. turning on his ally in the attempted conquest of Russia. (spoiler: neither campaign went well).
This may be a turning point that allows Napoleon to remain in power.
Questions:
How does this new government survive British opposition and presumed domination of the seas around Ireland (no doubt Irish trade would be savaged by the Royal Navy and Irish cities bombarded)?
Can Britain really contemplate an invasion of Ireland with tens of thousands of French troops backing a majority Catholic population armed to the teeth by Napoleon?
What happens to the Protestants whom have kept down the Catholics for so long?
Who, if anyone, will marry the 18 year old (and fairly hot if the pictures on Wikipedia are accurate) Queen Maria Beatrice?
How would the Pope react as Rome had ceased to recognize the House of Stuart as rightful Kings of Britain and Ireland (and His Holiness had his own issues with Bonaparte)?
Any ideas?