I don’t know of anything specifically in the Brehon Laws about burning trees. The Bretha Comaithchesa categorises the trees according to rank and deals with the various fines for damaging them, now that might include burning, but I don’t think it’s mentioned specifically. I think the Lay of Iubhdan is doing something else, although I’m still not sure what; however, I think it’s more than just a good guide to firewood.
As for the reference to the Silva Gadelica, you might be right, but I don’t remember coming across it. Do you remember where exactly it may have been given as a reference? I hope I haven’t overlooked anything.