This style of stirrup is known as an "Estribo de Aro" or a hoop stirrup.

They are made of wood bent into a hoop.

There is a leather strip around the outside of the stirrup and across the top of the arch.

Each stirrup has a white metal suspension ring at its top.
The leather is decorated with brass studs and flowers.
One of the stirrups has an old repair to its sole and has lost the leather on one side of its top.. 
There are no maker's marks or stamps.

Dimensions : Width of stirrup 1 6¼" (16cm) Stirrup 2 5½" (14cm), height 6¾" (17), depth of tread 1⅛" (3cm) , stirrup leathers 16" (41cm)
Weight : 507g (1lb 2oz)

Ref : stirrup415