The Shamin were designed by Fred Phillips and Robert Fletcher for The Motion Picture. With the approval of Gene Roddenberry, the Shamin, as well as numerous other new species in the film, were named by Fletcher and provided a backstory by him. The following is a brief description, in Fletcher's own words:
"SHAMIN PRIEST – From O'Ryan's planet, discovered by Paddy O'Ryan in 22nd century. Costume: made of fabric and liquid plastic, solidifies, turns into another kind of material. Bob developed this process more fully for the movie. All gold objects of this process. Rag parts of costume specially worn on hand looms." (The Making of Star Trek: The Motion Picture, p. 132)
"This person comes from O'Ryan's planet discovered by Paddy O'Ryan in the late 22nd century. The planet, like its people, are in an early stage of social and technological development. It will be a long time, if ever, before they reach the capabilities of space travel. Their society is like the society of America's western Indian civilizations. The smiilarities are particularly striking when one realizes that no travelers from Earth, in recorded history, had ever set foot on the planet until Paddy's discovery of it." 
In an interview appearing in the March 1980 edition of Fantastic Films, part two of "The Star Trek Costumes", Fletcher revealed more information about the Shamin priests and the design of their costumes:
"This is the Shamin Priest from O'Ryan's planet. His undergarment is of a knit material. The tunic is a very plush material that I had woven on a hand loom. The material can't be purchased commercially. It was originally to be used in an outfit for Mr. Spock, sort of a ceremonial attire. That didn't work out so the Shamin got it. At first they wanted him dressed entirely in rags but I didn't feel that he should be all that raggedly looking wearing all this fine gold jewelry, so this is the compromise we arrived at."
Maggie Browne added:
"The Shamin's mask is another of those resin and felt creations. This one is gold plated. The mask is really gorgeous but it had a lot of bad luck. It got broken twice because someone on the set sat on it twice."
In an article appearing in the January 2002 edition of Star Trek: The Magazine Volume 2, Issue 9, entitled "Who is that Alien?", the following information about the Shamin Priests, which included information from Fletcher's original background notes, added:
"We are provided very little information about the Shamin Priests. The notes simply say that they were from O'Ryan's planet, which was discovered by Paddy O'Ryan in the 22nd century."
According to Star Trek: Star Charts ("United Federation of Planets I", "United Federation of Planets III"), O'Ryan's Planet orbited a G-class star in the Alpha Quadrant. O'Ryan's Planet had, by 2378, become a Federation member.