was there any militancy related to them?
Yes, Liberation Tigers of Tamil Eelam (LTTE)
Sri Lankan Tamils
By the end of 1987, the militant youth groups had fought not only the Sri Lankan security forces and the Indian Peace Keeping Force also among each other, with the Liberation Tigers of Tamil Eelam (LTTE) eventually eliminating most of the others. Except for the LTTE, many of the remaining organisations transformed into either minor political parties within the Tamil National Alliance or standalone political parties. Some also function as paramilitary groups within the Sri Lankan military.
how many tamils are there in sri lanka 70% since how long have they been living here? Sri Lankan Tamils have a very long history in Sri Lanka and have lived on the island since at least around the 2nd century BCE. are they genetically linked to other sri lankans? Yes,  genetic studies indicate that they are closely related to the Sinhalese ethnic group in the island. which religion do they follow? The Sri Lankan Tamils are mostly Hindus with a significant Christian population.
wiki:15781797
Sri Lankan Tamils [SEP] Rise of militancy
Since 1948, successive governments have adopted policies that had the net effect of assisting the Sinhalese community in such areas as education and public employment. These policies made it difficult for middle class Tamil youth to enter university or secure employment. The individuals belonging to this younger generation, often referred to by other Tamils as "the boys" ("Podiyangal" in Tamil), formed many militant organisations. The most important contributor to the strength of the militant groups was the Black July massacre, in which between 1,000–3,000 Tamils were killed, prompting many youths to choose the path of armed resistance. By the end of 1987, the militant youth groups had fought not only the Sri Lankan security forces and the Indian Peace Keeping Force also among each other, with the Liberation Tigers of Tamil Eelam (LTTE) eventually eliminating most of the others.