There's a lot of dispute about how to make an "authentic leine" so I will briefly summarize the information I've been collecting. .
A leine (plural - leinte) is the basic unisex garment of the insular Celts of Ireland and Scotland, worn underneath everything else. It can be variously described as tunic-like, peplos-like, or some sort of chemise. It does seem to have been composed of two long rectangles of fabric attached at the shoulders either by seams or pins, with or without sleeves, gussets or gores. Necklines could be round, square or v-shaped, guessing from illustrations in the Book of Kells, but boat neck and slit-front are not out of the question. .
Early period (up to at least the 12th century ce) leines seem to have been either sleeveless, or with fairly narrow or straight sleeves eased by a gusset. Dunleavy states that styles gradually became more fitted over time, perhaps from exposure to Norse styles. .
Gores appear to have been speculative before the 10th century - they might have simply hiked their leinte up over their belts when they wanted to move at more than a crawl. The big baggy sleeves many people are familiar with are a later fashion, though quite fun, and there's no reason not to make your leine with these sleeves if you want to, they'll just be 16th century rather than early period. .
Quoting from "Irish Noble Dress: 5th century B.C. to 17th century C.E." by T.H. Lady Lughbec ni Eoin (Nancy Lynch) .
Styles and fashions of many differing centuries were worn side by side throughout the Middle Ages in Ireland. The Gaeils, being proud to display their connections to the past, traditionally honored their lineage by wearing garments to recall ancestral heros or events. Up to and including the Anglo-Norman period, just about any Irish garment style that was worn in earlier periods would have been worn by some Irish noble person. After the 12th century there were many confrontations regarding this custom and all other "things Irish" between the Anglo/Normans who had invaded the island country, and the Gaeil.