The so-called "Celtic" sorts of armor that show up in the historical record are very spottily recorded because the Irish and the Highland Scots were not prolific writers of records and pretty much behaved as preliterate, tribal societies anyway. What's in the "Celtic Fringe" of NW Europe, dating forward from about the fifth century AD amounts to some figural stones with reliefs and occasional runic inscriptions, either memorial stones or stone crosses of the Celtic type which sometimes include pictures. Then there is a gap of centuries, and stuff starts showing up again in the fourteenth century: a sculpture of an armored man in 14th-c. plate with a jupon over the body armor and a strange, awkwardly-shaped helmet with a barbute sort of face and the rest of which looks rather like some SCA spun-top from the 1970s or so.
The most numerous examples all conveniently gathered into one place are at the Tombs of the Kings on the Hebridean island of Iona. Numerous funerary slabs believed all to be of 14th-c. date all show variations on a single theme: an oddly tall bascinet-type helmet, no visor, and a knee-length textile
cotun, a style of gambeson. Distinctive Highlands/Islands swords equip these men, showing the Celtic fondness for angled crossguards such as on the large claidmh da laimh, a/k/a claymore, but on single-hand, broadsword-length weapons. Also the pommel forms may be either the ring pommel, with the end of the tang visible through its middle, or a late, stylized survival of the Viking-era 5-lobed pommel, now curved up to give room for the heel of the hand, perhaps for a more mobile manner of swordplay. These forms are a couple centuries yet before the basket hilt really began taking hold in the latest sixteenth and on into the seventeenth, as swords incorporating protective ironmongery became increasingly the fashion.
You can find more on the 'Net by searching for "cotun" than for "Tombs of the Kings Iona," which tends to frustrate. Likewise this site. There are pic links of gambesoned, tall-bascineted warriors of quaint simplicity. And nary a bit of interlaced strapwork on any of them. Maybe fighting was thought irreligious because a whole lot of the interlace art is seen in religious applications -- but none on the figures. It was also much less in fashion by the cotun era, whenever that got started. There are numerous memorial slabs with just a sword (Highland angled crossguard again) and some Gothickish ivy floral tracery as the entire decoration. Not even a name inscribed. But handsome withal. I'd want to have one for my headstone.
Turning to the better-armored Lowland Scots, what you find was they were still an impoverished, thrifty bunch, and their armor in use might be thirty to fifty years behind the latest stuff farther south in England -- which had a lot more money. Aside from the date gap, what the Lowlanders wore in battle was what the Sassenachs did. Harness was harness, enough said.
For the arms and armor of the Irish, it's difficult to find much. About the easiest thing to discover, aside from what is hinted for the late fourteenth/earliest fifteenth in Jerpoint Abbey's sculpture, is that tall helmets were popular with the Irish as with the Scots. Both used a helmet called a
clogaid, which seemed anything from a tall bascinet to a pointed-topped, finial-decorated affair -- Jerpoint Abbey's figure has a fleur de lys -- presumably devised for psychological effect. The other easy note to make is that the five-foot claidmh da laimh seems to have developed for the use of Scottish mercenaries known as
gallowglasses to cut Irishmen up with. The weapon's relative lightness and emphasis on cutting rather than an estoc's stiff section and thrusting tactics tells us armor was rare indeed among the Irishmen the gallowglass went up against. {ETA} Contrast this cutting blade with the heftier blade section of the German zweihander, which often had a flattened-diamond cross section, featured a sharp point and a leather-covered ricasso every bit as long as the actual grip, a large elaborated crossguard, and a secondary guard where the cutting edges started, called "parierhaken," or parry-hooks. This roughly-six-foot weapon was designed as an armor cracker.
Down in Wales, the warriors were less known for armor than for their most impressive weapon, the heavy-drawing longbow. Their last ruling prince, Owain Glyndŵr, was head to foot in the very same plate his English opposition wore, no doubt imported from the same sources. But in the Welsh context it took a magnate like Glyndŵr to come up with that much cash.
The rest of the Celtic Fringe is mostly Brittany, colonized from Cornwall and so proudly independent and distinguished that they still wave their own flag around at the smallest provocation --
Barruly sable and argent, canton ermine, and offer ermine-spot décor and triskele patches to the tourist trade. They make a mint too. Even the bank logos adopt triskeles.
Thus, the Celts.