Once more, I'll just toss out that saffron is not a specifically Irish usage, but was noted as the common color for the dress of Celtic tribes on the continent as far back as Julius Caesar. So, while the modern kilt is a Scottish adaptation, the use of a saffron kilt is a perfectly good pan-Celtic way of dressing.
In 1900 Seamas O Ceallaigh of the Gaelic League asked Pádraig Pearse about developing an Irish national dress, perhaps based upon a pair of trews which had been found at Killery, County Sligo dated from about the 16th century in the Royal Irish Academy Collection in the National Museum in Dublin.
On October 26 Pearse wrote:
"...one would at first sight take them for a rather clumsily made and ill-treated pair of modern gentlemen's drawers. Frankly, I should much prefer to see you arrayed in a kilt, although it may be less authentic, than in a pair of these trews. You would if you appeared in the latter, run the risk of leading the spectators to imagine that you had forgotten to don your trousers and had sallied forth in your drawers."
The ancient dress of Ireland was the leine and brat, somewhat resembling a woman's chemise and a horse blanket. As a modern form of national dress, it suffers from the same limitations as the trews in the National Museum. As the Irish leine was traditionally dyed the color of saffron, the color saffron was chosen as the color if the Irish kilt. There is no evidence of kilts being worn by the Irish, saffron or otherwise, prior to 1900.

As the Celts were tribal, spread over a wide geographical area, never unifying into nation-states and with great diversity in religion, language and customs, I find the possibility that they dressed in a common color rather remote, unless you have some supporting documentation.