Crochet sizing down irish crochet pattern for 18" dolls

I usually change sizes with different needle and thread. Going for a slimmer thread and smaller gauge needle would help

 

This is very late, but:
This is a masterpiece of Irish crochet. If you're going to tackle it, I salute you!
The pattern is already using the finest thread available - size 100. So finding a finer fiber might not be possible. A single strand of silk embroidery thread might be finer. The look of silk will be quite different from the usual cotton.
Decreasing the center panel or the area near the side seams is a good idea. The great thing about Irish crochet is you make the separate elements first. So you can make the leaf and flower shapes first and evaluate their size before committing to the whole garment. If the elements are too large, you'll have to adjust their size by eliminating some of the rounds or rows. The mesh background will determine the size of the garment.
As a short cut, I'd recommend making an Irish crochet central panel sewn to a fine cotton fabric garment. Frankly, I'd buy a lace panel that looks like Irish crochet and use that, unless you're a very serious crocheter!
Best of Luck!

 

Your comments are wise. I wonder if some kind of cotton mesh could be used for the background and then then simplfied motifs could be worked in 100 and sewn down. Lacis would be the first place to look for the mesh.