This must be the worst instance of copying from Yuvan! Song in question: "Karuppaana Kaiyaale" from "Thamirabarani". This was obvious right on my first listening - the original song, being the famous devotional song "Karpoora Nayagiyae" by LR Eswari.
Listen to the songs here:
Yuvan's "Karuppaana":

"Karpoora Nayagiyae" sung by LR Eswari

I had heard this song quite sometime back but only recently, came to know that the culprit was Yuvan. I assumed the composer must be Shrikanth Deva (who had shamelessly lifted the MGR song, "Koduppadhellaam Koduppaan" in one of his earlier movies). Never thought that Yuvan would stoop so low.

Agreed... Yuvan is no original composer, with many of his famous hits (including the 3 songs in Kadhal Kondein) being blatant lifts - but in those cases, it was only the first few lines and the sources were pretty unknown in India - they ranged from Indonesian to Nordic folk. But, here the source is a very famous devotional song, and the entire song is copied. Despite his copycat reputation, I had a little respect for him, but with this song, even that is gone. To me, he is now on par with the Shrikanth Devas orDhinas of the world!!!

PS: The song "Karpoora Nayagiyae" itself is inspired from the Navrang song Aadha hai Chandrama, but in this case the similarity ends after the first 4 lines.
Listen to Aadha hai chandrama, here: