I have a theory. The One, worshipped in Iri and Rira, is Virtuosity. There are a few points that make this make sense:
1. The One is said to have split itself apart in pursuit of new experiences. Virtuosity splintered herself, and I suspect it was to better experience and inspire art. She, according to WoB, spent her time pre-splintering viewing art throughout the Cosmere. This lines up well with the One's goals. The pursuit of new experiences likely manifests itself as artistry in Iriali and Riran culture, so the One and Virtuosity do have similar Intents.
2. The Iri are not native to Roshar. They could be from an area more influenced by Virtuosity, but then left, carrying their religion with them. There is the question, however, of what might drive them to leave. What if the Iri were the natives of UTol, and fled when the Fain arrived? It's pure conjecture, but it would make sense.
3. Brandon has said that the One is not Adonalsium, and that Iriali religion is more influenced by more recent events. Perhaps the splintering of Virtuosity?
4. This is the most dubious point, but it could work. In Yumi, we only see cyan and magenta hion lines. Brandon has hinted that yellow may also be connected to Virtuosity, as a third color in the triad. What if that is what gives Iriali and Riran people their blond hair? It doesn't really match how hair color normally works, as Adolin has half-blond half-black hair, instead of a combination. In addition, Iriali religion says that all people are fragments of the One. This particular belief could come from all Iriali and Riran people being Invested with Virtuosity's power, which manifests in their hair color.