It's all 18/3 Barbour, but it used to be made in Ireland before the company shut down. It's now made in the EU somewhere. There's still the original spools out there but they're not always stored correctly, and can be very weak and brittle.
You can do 2 ply Flemish but it won't be very strong. It can get quite complicated, but essentially you need to make sure you have a maximum of 7 individual strands per bundle. Depending on breaking strain you then need to calculate how many bundles you need. 3 ply is far stronger than 2 ply, even with the same number of strands. For instance, if you decide you need 21 strands, you must make the string with 3 plies of 7 strands.
With modern string materials you can just bunch up as many strands as you want, but with natural fibre strings you have to be much more careful.