The Royal Barefoot Nuns, of course. If I had to guess, I'd say they are most likely to be discalced carmelites. I feel confident that Wordie already has an entry for the d.c's.
Anyhoo, las descalzas Reales live in the Real Monasterio de las descalzas Reales. Which I am happy to visit, in the spirit of inquiry that makes Wordie lo que es. But not until Wednesday, when entry is free to citizens of the EU. 5€ seems a leetle exthpenthive to me.