Archaeological excavations in rainforests of Sri Lanka unearthed earliest evidence for hunting with bows and arrows outside Africa (48 000 BP). The oldest remains of homo sepians date back to 34,000 BP are stone age hunter gatherers. The closest lankan group to these people today are the vedda people.
There are evidence of Iron age culture had came to Sri Lanka through southern india. Iron Age of Sri Lanka is between ~1000 BC–543 BC , in 543 BCE indo Aryan migration happen (arrival of Prince Vijaya). However there are archeological evidences existence of civilization 300 years prior to Vijaya’s arrival and the Tamils migration happened prior to establishment of Sinhalese identity and the ancestors of present Sinhalese migrated from eastern India.
The Indo Aryans and Indo Drvaidans are mostly the same in genetics while Indo Dravdians have bit more South Asian related DNA(ASI), while Indo Aryans have bit more steppe related DNA(ANI). You can see similar genetic differences between Lankan ethnic groups.
In this scenario the Sri lankan Tamils or Sinhalese are not the oldest inhabitants of Sri Lanka. The evidences however show both are equally native to Sri Lanka; while the Indian Tamils in up country areas are recent migrations from Southern India 19th to 20th century.