I've done a lot of research on the history of knitting, the earliest knitting was known in Europe (from Spain) seems to be knit in the round (probably Islamic artisans hired by the royal family - Islamic style on pillows for the burial of royal infants but with Christian and royal family symbols).
All the pictures I am aware of, of the Virgin (or others) knitting in the late 13th, 14th, and 15th century (mostly early 14th) show knitting in the round: shirts, purses, socks etc. My favorite is Mary knitting a "sweater" for the young Jesus who reads a book at her feet (about age 7, the age of reason in the High Middle Ages).
I've actually done a handspun, hand-dyed copy of that one year ago except I made the neck too small.
Anyway, flat knitting is certainly starting to happen a bit in the 16th century (when the knitting machine gets invented) and becomes the rage in the 1930s when tailored clothing and commercial knitting machines really came to be popular in a big way.
There is a huge historical debate I won't get into on the history of pearling when it comes to being "able" to knit flat and still get "garter stitch," I really have no opinion on that only that certain types of European knitting (like my German housemate does) do make it just about as easy to pearl as it does to knit, and I'm pretty sure creative people would have figured this out lots of different times.
If I had learned that style first, instead of later (and I can only do it when knitting two-handed) I probably wouldn't care if I was knitting flat or round either.
Finally: Gansey's probably ARE a native English/Island tradition and the ultimate expression of that shirt Mary is knitting for her son many centuries ago in that painting.
Hence, knit in the round (on huge long needles like fair isle in Scotland) only switching to back and forth to do the upper patterning that would show over a fisherman's overalls.
The patterning is also heavier and warmer than plain stitches are, and the plan stitches go under the overalls.
Aran patterned sweaters on the other hand, from all the REAL information I can source (forget tourist trap stuff), are not actually indigenous to the Aran Islands which has however adopted them.
Despite all the lovely mythology around ancient "Orish" fisherfolk knitting individual family patters to "identify the dead" who would neatly "wash up on the beach" after shipwrecks (how accommodating of them - sarcasm alert) in reality the sweaters seem to have been unknown before two sisters who immigrated to Canada when young moved back in their late middle age and brought the flat knit, patterned, cabled sweaters with them - from the Eastern Seaboard; where similar sweaters are also popular.
They are popular because slightly greased wool (washed but home washed) done with lots of cables and pattern stitches are WARM and perfect against the HIGH WINDS of the Atlantic on both sides of the water. They also make bulky and large pieces which frankly ARE hard to knit in the round unless you are knitting something smaller (like baby clothing) or do the two-person sweater a la Elizabeth Zimmerman and her daughter Meg.
It is possible to knit them in the round, but I wouldn't in Summer or anytime getting overheated is a bad idea.
I haven't made a full one yet, but after the round "sweater of way too many cables" that is still a UFO nearly 6 or 7 years after it started, if I do I will probably knit it flat unless it is a more simplified version.
Flat knitting allows for a LOT more shaping and tailoring which can be done by round knitting with steeks but is often easier done in pieces.
Anyway, it might be worth checking out to see if you can find a flat gansy pattern as a template (until the 1990s most patterns were made into flat patterns for the US market).
Purple Kitty has a lot of vintage patterns for free, though I sometimes buy their yearly wrap-ups. I would start looking at their lots of patterns from the 1930s through the 1980s. I use the children's knee sock pattern from the 1950s for myself (I've got kid shaped legs on a body the size of the average 9-year-old).
www.purplekittyyarns.com