Underwear is through most of all epochs a topic that is not mentioned in great detail. There are also no finds from the Celtic period, as even from the outer garments we at best find small fabric remnants. As always, archaeology suffers from the closeness to nature of the Celts, who largely used organic materials for all aspects of their lives.
Thus, we must approach the subject of underwear among the Celts through inference and logic.
Let us first consider male underwear.
Even among primitive peoples, it is quite common to wear penile sheaths or loincloths. Man's best friend is a sensitive thing and wants to be well protected.
From kilt wearers in my circle of acquaintances I know that a kilt "down without" can be quite uncomfortable. Whether the friction soreness is due to the fact that today's kilt wearers are simply not accustomed to no underwear, or if it is actually a fundamental problem, I can not answer as a woman, but certainly know the issue of rubbing thighs in summer ... nobody likes that, you try to find a solution for it.
We know that the Romans knew underwear. Men (whether some or all is debatable) wore loincloth-like pieces, a length of cloth that was put through a belt in the front and back. This is a form of underpants that was used for a long time.
The Celts, unlike the Romans, wore overtrousers called braccae, so their best part was protected anyway. As a riding people, they certainly attached great importance to that, at least I imagine it very unpleasant to ride on the horse without protection of the private parts. For them, underpants were therewith not as important as, for example, for the Roman soldiers in their skirts.
From the purely practical side - and this should always be considered - underpants would probably even have been cumbersome. Even if illustrations on situlae show the Celts in such tight pants that one is tempted to think of knitted or needle-bound materials, the braccae were probably made of inelastic linen with a binding band. Underneath, then, a loincloth with the fabric held in place by a belt - wriggling the best bit out of there probably made any urination a laborious affair.
My guess would therefore be that Celtic men did not wear underpants under the braccae.
And the women?
Women's underwear is even rarer than men's underpants, at least down below, throughout history (even though historical films try to make us believe that knee-length lace pants were always "in". If so, even these were mostly two tubes without connection in the crotch) Also this has quite practical reasons. In times when there were no toilets, but you relieved yourself behind the next bush, it was much easier without underpants. You just had to spread your skirt a little, squat down and let it run. The more voluminous and wide the skirts, the less likely that underpants were worn. Anyone who has ever worn a gorgeous wedding or ball gown knows how tedious it is to pull down your underpants under all that fabric. Without exposing your private parts to the light of day because you have to gather the skirt up to your waist, it's almost impossible to do. The only exception to this wide-skirts-no-underpants rule is the time of the crinolines. Here, underpants were worn, because a gust of wind could make the skirt tilt in such a way that the woman stood there naked.
What does this mean for the Celtic women? Their bodyshirt and peplos could be tight or wide, theoretically underpants would be possible, yet unlikely in everyday life. Roman women wore panty-like things (two triangles tied together at the sides) when they played sports. If we assume that it is true that Celtic women could also be warriors and rode horses, there is a distinct possibility that they wore similar underpants at least then. Or braccae under their clothes like the men wore.
(In connection with underpants and women, the subject of menstruation always comes up, which I will address in another blog article.)
Women's underwear, however, consists in our minds not only of the underpants, but also a bra. Again, we know that Roman women certainly wore breast binds. I assume that also Celtic women, at least those with larger bust size, resorted to such bandages when their breasts dangled uncomfortably loose. But even today, women are very different, some prefer the tightness of a bra, others like to swing freely.
And that is the beauty of the Celts: this culture consisted of many tribes, each of which had its own customs and rules. And the people themselves were much more individual and different in their garb, this "obstinacy" can already be read out in the descriptions of the battles. The Romans, uniform uniform, uniform behavior, the Celts a wild bunch.
Accordingly, we want to leave them their individual underwear secrets and just be aware as a modern person that briefs, thongs and boxers are something very new.