The range of borrowings goes beyond the scope to be expected for a situation where two neighbouring peoples exchange material goods: Firstly, there are many Tamil loanwords pertaining to everyday and social life (kinship terms, body parts, ordinary activities).
This - along with the impact Tamil has had on Sinhala syntax (e.g. the use of a verbal adjective of "to say" as a subordinating conjunction meaning "whether" and "that") - is suggestive of not only close coexistence but the existence of large numbers of bilinguals and a high degree of mixing and intermarriage.
The inversion of this argument is not possible since loanwords already matching the linguistic requirements of the target language may remain unchanged.
Thus, the word täpäl (Tamil tapāl) gives away its old age because the respective umlaut processes took place before the 8th century; iḍama (Tamil iṭam) however needn't be a recent borrowing, because no sound changes that could have affected this word have taken place in Sinhala since at least the 13th century.
Exceptions from the standard are the romanization of Sinhala long "ä" ([æː]) as "ää", and the non-marking of prenasalized stops.