Irish Celts in particular, I guess around 58 BCE when the Romans invaded Gaul?
I know the stereotype of them wearing blue, green and red tartan but how did they wear it? as tunics, separated pants and shirts? what about cloaks and shalls? Tattoos, hairstyles (For men) Jewelry if any.
Also what materials where they made of? how did they dye their clothing?
Your stereotype of tartans and tatoos sounds like Picts of Albion, not the Scotians of Ireland. Irish dress isn't what anyone expects. ;D This is your pagan Heroic Age Ireland. I used to have an SCA historical tailoring client who wore it, so I researched this.
The primary garment in Ireland was the leine or saffron shirt, made of soft but sturdy linen dyed with saffon and stitched with scarlet or emerald thread. It hung to the feet and had long full sleeves, probably cut similar to some African tobe of narrow linen, based on engravings. Men would pull it up through a belt to shorten it to the knee, sometimes with a belt over the fullness. They wore no trousers but went bare-legged and usually barefoot.
Over this would be worn the brat, a full cloak of a solid colour like red or green, of woolen or silk. It would be decorated with cords of coloured silk braided and knotted and sewn down, in some cases until one could barely see the ground colour. Naturally, the higher the rank, the thicker the decoration and the finer the materials, including golden ornaments and cords. The brat closed with a clasp at the throat.
They did not wear "Celtic knotwork" at this time. Garments were decorated with scrolled and spiral floral vines.
Jewelry included bracelets, rings, brooches, belts, and rigid torques (not like viking chain torques). Look for any rigid necklace with an opening you slip your neck through, paint it gold, and wear the opening in front. You can also put a rigid wire in a short length of gold rope and cover each end with some ornament.
If you don't want just long loose hair, a distinctive style was to divide it into three or seven braids, then cover the ends of the braids with hollow golden balls. You usually have to adapt some golden small Christmas ornaments for the job! Plastic is better than glass, and you usually stitch them above the rubber band with gold thread to hold them on over the ends.
If you have short hair like a slave, consider an inexpensive costume wig or go full hog with hair extensions.
Sources: P. W. Joyce, A Social History of Ancient Ireland (1968) 2 volumes. There is also a compact and more affordable version, A Smaller Social History of Ancient Ireland.