It is however a possibility that the inor/ionar was developped after the "warrior coats" of late 6th-7th century we see depicted for exemple on the Vendel helmet plates. We see something very similar to the inor on pictish stone carvings in the 8th century, and later on 9th century Dal Riada art (both manuscripts and carvings). There seems to be both short (the most common in irish art) and longer version.
It's basically a sleeved coat.
Now there is hardly any depiction of irish or pictish garnment before the 7th century at earliest, so it could have been developped on it's own. But we see so much influence from that period onwards from Anglo-saxon Northumbria (and to a lesser extent, Frankish Gaul) in Pictish and Irish art (like knotwork), metalwork techniques (chip-carving for exemple), weapons (sax from Lagore, sword pommels), religious matters (with the progressive acceptance of the roman easter dating) that it is also much likely new dress fashions could have been adopted. On a similar topic, we see gussets depicted on pictish tunics and I think those gusseted tunics could also have made their appearence by then.
See this article on pictish dress:
http://eithni.com/referencedesk/TheWell-DressedPict.pdf