Sinhalese Nagas 
It is very much founding fact that Sinhalese were unification of hela groups and sinha. Also it is very much famous fact that there were 4 hela groups in the island. Origins of the Hela Race. Hela was combination of Raksha, Yaksha, Deva, Naga. 

Raksha were extreamly powerful in the history but may be lost power due to battles,.. At the time of start writing the history, Raksha and Deva wasn't powerful. Naga was the most powerful authorities. That time only they had kingdoms. It is evident that they ruled two kingdoms Kelaniya and Jaffna. Others lived as small groups ( kuveni's story). 

Rajarata area was ruled by Yakshas before Vijaya. After 2,3 generations from Pandukabhaya, kings tend to suffix 'Tissa' and 'Naga' to their names. Tissa name is comes from Sinha tribe. There is no reason they added Naga to their names other than representing Naga tribe. This says later Naga royal families in one of the 2 kingdoms ( most probably Nagadeepa) made connections to the royal family in Rajarata. Nagas came to Anuradhapura and joined the Sinhas and Yakshas and actually able to dominate over others. So kingdom of Nagadeepa was lost from the history and Rajarata was trying to gain Naga/Sinha identity in which there had been a Yaksha rule. But kelaniya was there and ruled as maya rata.  

It is also very much evident Naga were the dominant tribe withing the core of the kingdom to protect their king. Those days without having clear power king can't survive. Yaksha's had clear control of Rajarata during the period of Pandukabhaya. But later Naga influence of the kingdom started to growth due to connection between royal families. Since conflicts between tribes hasn't been ended, Naga/Sinha king wasn't be a fool to rule middle of Yaksha on those days. Also we can see there is a clear discrimination against yakshas and had introduced a superiority on Nagas defining solosmasthana. Also some Yakshas leave the Rajarata and come other areas. Ritigal Jayasena was one of them.

So Nagas dominated over Rajarata and Maya rata but still others may had the control in Malaya rata and Ruhuna. SL was unified on tribe basis. After SL was converted to buddhism it wasn't required to unify tribes which based on religion. Since Ruhuna also accepted Buddhism (due to fear for Asoka) Rajarata king could made his brother ( MahaNaga) as the king of Ruhuna without considerable resistance. So it is very much evident that Sinhalese mainly consist of Nagas.