All Sri Lankan Muslims are not of Dravidian origin. Some are Malay Muslims, others are Moors; but the majority are indeed ethnically the same as the Muslims in Southern India - so perhaps of Dravidian origin. That said, I would still agree with your point that Tamil and Muslim are not ethnic classifications and it is illogical to divide the Sri Lankan population as Sinhalese, Tamils and Muslims (and Burgers as sometimes portrayed) - which mixes up linguistic and religious classifications (not providing for Tamil Muslims or Sinhalese Muslims for instance). Linguistically there are basically only two major groups - Sinhalese and Tamil (though Malay and Urdu might be spoken at some of the Muslim homes, most of them speak Tamil as well). Religiously there are 4 groups: Buddhists, Hindus, Christians and Muslims. Theoretically any of the two linguistic groups could belong to any of the 4 major religions. So, why do we divide ourselves into so-called racial or ethnic groups? Very unproductive, illogical and useless really. We should just call ourselves Sri Lankan, regardless of religion or language.