"Saffron" colour of Irish kilts?

[O'Callaghan]

But when it came to a kilt, they were of course to be wool.

That was exactly my meaning. Anyone dyeing a woolen kilt with saffron would necessarily have wound up with the shade that we now have.

I have not come across anything that would say that brats or mantles were not dyed in saffron.

Indeed not, although apparently the only quote on the subject says that brats were 'striped'. Some take that as tartan, which may or may not have been right. There is certainly much earlier evidence of tartan being known to celts. OTOH, neither did it say striped vertically and horizontally, so they may not have been tartan either.

We won't get into the whole discussion of Irish and kilts, that has been done many times. None the less, there has been 110 years of "brownish" saffron kilts being know as the "Irish tradition".

As of 1900, the discussion seems to have centred on adopting something for Irish national dress that would actually get worn, not on accurately reproducing something ancient.

Nevertheless, there are some reports of the British Army adopting the saffron kilt for Irish pipers at some point during the previous century. Whether the British Army thought that the ancient Irish had worn kilts is not something we know or are likely to find out. What we do know for sure, though, is that the kilt could be combined with standard military uniform on the upper half of the body, whereas that isn't true of the leine.