When I get questions about yarn weights, I always feel a bit lost since we don’t group yarns this way. Now I stumbled on to this guide on craftsy.com to determine your yarns. I have tried the method on the yarns from Sirri and Snældan, and here is the result.
WPI is Wraps Per Inch, and as you can see I wrapped the yarn around a pencil.
Sirri 3-ply: 12 WPI = DK (Doubble Knitting)
Sirri 2-ply: 20 WPI = Fingering
Sirri 1-ply: 30 WPI = Lace
Snældan 5-ply: 10 WPI = Worsted
Snældan 3-ply: 17 WPI = Sport
Snældan 2-ply: 22 WPI = Fingering
Snældan 1-ply: 36 WPI = Lace
At least this should make it very clear, that 3-ply Sirri is not the same weight as 3-ply Snældan.
There are of course different terms from country to country, so here is a link to Yarn Weigth Terms to help find the right terms and which needles to use.
Hope this was helpful, and comments are always welcome :)