There were two types of cloak worn by the Celts.

One was a 'bad weather' cloak that was heavy, lined, and hooded.
(See Celtic Clothing )

The second, as shown here, was lightweight and more for decoration. It also helped to keep out a cold wind. It was small enough to to made in brighter colours and more elaborate patterns (both of these take skill and time, and were expensive!)

The warrior in the back ground is wearing a cloak made of skins, and nothing else!