The earliest existing Claddagh rings all date from the 1700s, though there is some evidence that they were being used before. It is only the solid gold jewellery that has survived and they all bear a crown.
Of course, the impoverished Irishmen wouldn't have been able to afford such fine metal. Their rings would have been cheaper, hand-made and lost to history. How many of them would have shown just the heart and hands alone?
Ireland entered the 18th century having suffered defeat at the Battle of the Boyne. The British were in charge and with them came the Penal Laws.
In a country where the majority were Gaelic speaking and Catholic, these two attributes set them at a disadvantage. No Catholic could legally vote or hold office. The language of government was English. By the end of the century, the entirety of Ireland had been divided between British land-owners.
For the native Irish, allegiances became a matter of life and death. They did have the choice of converting to Protestantism and learning English. They would have to raise their children to disdain all things Gaelic, despite their own ethnicity. Even then, opportunities would remain limited with preference in all things being given to British workers emigrating to Ireland.
Those Irish who opted to keep their own language and culture were punished. Their land was seized and they had to pay rent to their new English overlords. The cost was kept high enough to plunge the Irish families into abject poverty.
Naturally the Irish continued to fight back. This was the era of the Whiteboys and Wolfe Tone. It was a century that ended with a massive pitched battle, between the English and Irish in Wexford. The spirit of rebellion never did die, for all that the British did in retaliation, and eventually the country was won back as the Republic of Ireland.
The Claddagh rings became highly popular during the backdrop of this bitter 18th century struggle. Those men adding a crown to their rings were stating loud and clear that they supported British rule. The Fenian Claddagh rings were worn by men prepared to starve or die, living the rest of their lives in poverty and persecution, rather than give up their Gaelic heritage. They did not accept the right of the British to rule over Ireland. They did not wear the crown.