The problem with #20 thread and Irish crochet is that the work becomes too unmalleable. Joins are less neat and certainly more noticeable. Also, a motif is not as versatile in larger thread. The beauty of Irish crochet is that it has alot of delicate detail. This is lost when large threads are used. In short, what works with a #40 or #60 and finer will not work with a #20, let alone #10 (Ew!! Shudder at the thought of using #10).
Various Thread Sizes
Vintage MotifThis is a vintage motif worked in a very fine thread. Click on the link to see a close-up image. It is only 2" tall but just look at the detail. The background has Open Clones Knots.Each one is about 2 mm long!!! Click here to watch how I make them.
This image is the same vintage motif and another motif I made of the same pattern. I used a #60 thread and crocheted this motif by eye as I don't have instructions for it. If I had to guess, I'd say the vintage motif is crocheted in a #180 thread (though I won't swear it).