If you have already discussed this - then leave it. There is a reference in Appar Thevaram - 6th Thirumurai about the darumi inccident. Appar on entering Pandinadu, worships at Tirupputtur, (sivaganga dt) Thiruttalinadar temple. He remembers the inncident and sings in Tirupputtur thevaram. Since he clearly states about sangam, the legend was there in 6th century itself.
நன்பாட்டுப் புலவனாய்ச் சங்க மேறி நற்கனகக் கிழிதருமிக் கருளி னோன்காண்
( full song can be referred from site given below - 3rd song of the padigam) http://www.shaivam.org/tamil/thirumurai/thiru06_076.htm So, Sangam, Dharumi and Irayanar inncident were clearly mentioned. Only Nakkeerar name is not mentioned.