That makes sense, I guess somewhere in my self-education I missed the rule about lenition indicating a definite noun. Lenition in general seems to be the most regionally variable aspect of the Irish language, so I tend to treat certain examples as I encounter them with a grain of salt, chalking them up to dialect, but in this case it looks like I have missed out on a fine point of grammar. But it makes sense, especially knowing already that names are lenited (i.e. teach Sheáin). The older construction (solais na gréine) is the one that seemed more intuitive to me, but it looks like I could have probably got away with some of my dignity if I said that in conversation (and I am sure your hypothesis about the development of that is correct). Thanks.