The Celtic brooch, more properly called the penannular brooch is a type of brooch clothes fasteners. They are especially associated with the beginning of the Early Medieval period in Ireland and Britain. They are the most significant objects in high-quality secular metalwork from Early Medieval Celtic art.
How to fasten a pennaular brooch:
The pin is pushed through folds of the cloth, which are then pulled back inside the ring; the free end of the pin passes through the gap in the ring. The pin is then rotated around the ring by 90 degrees or so, so that as long as the pin is held down by slight pressure it cannot escape over the terminals, and the fastening is secure