Alrighty, well first off the Celtic symbol most prominently shown in the game is the Celtic knot, which itself doesn't have much meaning other than being an elaborate decoration. It is used, however, to accentuate the importance and promote various themes within a work, the most well known being the Illustrative manuscripts in real life. In game the importance of the tapestries found in the homes and sometimes tattooed onto a persons body is highlighted with numerous Celtic knots.
Talking about tattoos, Albion, which is an ancient and sometimes poetic name for the British Isles, is shown to have a long cultural familiarity with them. The same is true with Britain, which if you have even a passing familiarity with Celtic history you might remember the Picts, a nickname given to them by Roman soldiers derived from the Latin word "Pictus" meaning "Painted". It isn't clear whether this meant that they painted themselves literally, some say with dyes derived from woad, or if the description was referring to tattoos possibly made from iron oxide. Tattoos predate the Celts by several millennia, so it isn't out of the question. Either way, tattoos featured prominently in Celtic culture, being described at least as far back as the first century B.C. by Gaius Julius Caesar. In his commentaries some Celtic warriors are described as wearing nothing into battle except paint(maybe tattoos) with designs of wild animals and faces covering their entire bodies. Some of these descriptions clearly inspired some of the villagers of Knothole Glade, especially the Chief and the Tattooist himself.
Other examples of Celtic symbolism involved the spiral, in itself reflecting growth or development, but often used contextually with other symbols to reflect an idea. Oak trees symbolized the center of all things, countless myths and legends include them and because of this they symbolize a wide range of things but otherworldly strength, to divine wisdom. Oaks were also know as the gateway to the Otherworld (fairy realm), a feature of Celtic religion which involved the belief of a kind of reincarnation in which you never die, you simply pass to the Otherworld. On Knothole Glades Tattooist's stomach we find an Oak tree with spirals curling outwards from its branches, which if the devs had any real meaning in the design then it might've been a never-ending growth of strength, skill and will, the widely varying qualities in which the Oak reflects.
Still we can find more Celtic influence within Knothole Glade. Ancient Celts had embedded music deep within their culture to such an extent that some of the oldest musical instruments in the world come from prehistoric Ireland. Some instruments, such as the carnyx, were even used in battle to relay orders to troops and intimidate opponents. Right at the gate of Knothole Glade we find what appears to be a massive carnyx attached to a podium sitting atop a hill. While the use of this specific carnyx is a bit different from those used in reality it is still prominently featured, displaying to all passersby the magnificent cultural artifacts passed down to the current inhabitants. Wood carving is noteworthy here, as well as in the totems found around KHG, since much of Celtic society relied on wood for the transfer of information and for decoration (a fact that lead most Celtic artifacts to be irrecoverably lost to time).
Celtic language is something largely wiped out from history, the late Celts would usually use the Greek or Latin alphabet to carve out inscriptions, but there are examples of their own Celtic alphabet known as the Ogham alphabet, though this never developed past its infancy. Aside from the Ogham letters are more traditional glyphs, or simple stylized pictures symbolizing an idea or a thing. The largest of these are carved out on hillsides, known as hill figures. Famous ones include the Uffington white horse and the long man of Wilmington. A mix of Greek and Ogham letters and glyphs reminiscent of some of Britain's most famous hill figures are displayed on the tapestries found in the homes of villagers in KHG. There are lots of cultural inspirations, not all from Celtic society, which shaped Albion, but Knothole Glade especially is the home of the Celts, and that's just perfect to me.
If you have any more questions lemme know. I'll possibly get back to ya if I have time.