From A Wiki of Ice and Fire
The Rhoynar lived in city-states along the vast network of the river Rhoyne until 700 years before Aegon's Landing, when the Valyrian Freehold's expansion from the east threatened to overtake them. Prince Garin led 250,000 Rhoynar to their deaths trying to defeat them.
A warrior-queen named Nymeria united the city-states to flee across the narrow sea to Westeros. According to myth, she guided ten thousand ships out of the Rhoyne Delta, into the Summer Sea, west along the coast and through the Stepstones, to finally land in Dorne along the coast and the banks of the Greenblood River.Citation needed Nymeria married a Dornish lord, Mors Martell of Sunspear, and helped him consolidate Dorne under his rule.
The Rhoynar have lived and intermingled with the Dornish since that time. Before their flight from their homeland, they taught the Andals how to work iron, although in Westeros, the Faith of the Seven teaches that the Andals received this gift from the Smith in a vision. Citation needed
During and after the Rhoynar evacuation, the Valyrians destroyed the Rhoynish capital, Ny Sar, and many of the other great cities of the kingdom, including Chroyane, Ar Noy, and Ghoyan Drohe; their massive ruins are still easily observed from the Rhoyne
The Rhoynar were a slender people with smooth olive skin, black hair, and dark eyes.
The Rhoynar practiced a number of customs that differ from mainstream Andal culture. They practiced equal primogeniture, granting inheritances to the eldest child regardless of gender. They also tolerated homosexuality. Due to their origin in city-states, the Rhoynar titled their rulers Princes and Princesses. Their influence made these customs standard in Dorne as well.
Other aspects of Rhoynish customs were abandoned in favor of Dorne's Andal culture. The Rhoynar originally worshiped a number of river-themed nature gods. Their primary god was Mother Rhoyne, or Mother River, the personification of the river Rhoyne itself. Other gods include the Old Man of the River, a turtle god, and his adversary the Crab King. Having abandoned their river, however, the Rhoynar adopted the Faith of the Seven.
Some descendants of the Rhoynar did not assimilate and continue to practice the traditions of their ancestors. Called the "orphans of the Greenblood", these individuals live on rafts along the river Greenblood and consider themselves orphaned from their Mother Rhoyne.