Identifying the timing (thalam) of a song is always very interesting, especially songs with a bit complex rhythm. I remember arguing with my colleagues that "Roja poo aadi vandhadhu" song from Agni Natchathiram is in 3/4 or some multiples of it, where as they were arguing that it was in 4/4. You can actually count 1-2-3-4 with that song and it will actually fit into the song rhythm. Only after a few days, did we conclude that the song was in 6/8 (tha ki ta ; tha thi mi). Other similar songs in 6/8 include Iravu nilavu (Anjali), Vegam Vegam (Anjali), Kaalam Kaalamaga vandha (Punnagai Mannan), Theme Music (Punnagai Mannan), Vaan Megam (Punnagai Mannan), Oh Oho Megam vandhadho (Mouna Raagam), Aathadi Ammadi (Idhayathai Thirudathe), etc. Interestingly IR has composed the last 3 songs, which are actually rain songs, in similar rhythm. Infact 6/8 rhythm generally conveys / brings in a jolly mood. I also heard that IR has composed many of his songs in 6/8, while ARR uses 4/4 by default. But I guess a composer will not generally start composing by saying, let me do it in 6/8 or 4/4 or whatever. It comes natutally to them when they start composing. It would also depend on the environment and culture in which they were brought up I believe. IR would have probably heard a lot of folk songs in his childhood, and hence 6/8 might be natural to him.
OK, now try identifying the thalam / timing of the song "Malarae Malarae" from the film, Power of Women and post it in the comments. The rhythm pattern is not a normal one for this timing and hence might be a bit confusing. It is a good song by Vidyasagar with a strong kaapi flavour, and sung by Hariharan (he is the lead artist too in the film).