It follows a two-day trip to Northern Ireland, which began in Belfast on Monday. The Duchess joined Prince Charles on Tuesday.
The couple's first stop on Wednesday was to Donegal town and the factory where the famous Donegal tweed is made.
Their visit to Donegal was at the request of the Irish government.
Sean McLoone manager of Donegal Castle said the visit would provide a welcome "tourist boost".
"It went very very well, the sun shone which always helps and I thought the castle looked resplendent and certainly the royal highnesses expressed their thanks, they enjoyed their tour.
"There was a lot of smiles all round. I think its very important from a tourist perspective because a lot of our visitors would be traditionally from England anyway," he said.
Róisín McCool, principal of Gaelscoil na gCeithre Máistrí, the only Irish speaking school in Donegal town, said it was an honour to perform.
"We were absolutely honoured and privileged to be here today to be singing and dancing and playing music for Prince Charles and Camilla.
"It went fantastic and they loved it. Everybody was so down to earth he was absolutely lovely and he was talking to the kids and us.
"The kids sang their hearts out and they played so well and danced fantastically well. So I was over the moon and delighted for them all," she said.
After a visit to Donegal Castle, the party made their way to the Donegal Tweed factory where the Prince and Duchess were shown how the fabric is made.
They finished off the trip with a visit to a local butcher shop to sample some produce.
Then they were received at a civic reception at Letterkenny, before taking a tour of Glenveagh National Park.
It was the Royal couple's second official visit to Ireland.
Last May, they visited Mullaghmore in County Sligo.
The Prince of Wales and the Duchess of Cornwall are on their first official visit to County Donegal in the Republic of Ireland.