Don't be fooled by the unpretentious exterior at this Irving Park spot--within is a lavish Thai restaurant. Many intimate dining nooks and small rooms fill the two-floor restaurant; each is appointed with works of art, including paintings by owner Arun Sampanthavivat's brother. What his brother does with a paintbrush, Arun exhibits in the kitchen. Dinners come in one form only--a multi-course degustation ''chef's design menu'' for $85. You will be asked about allergies and spice-level preference, but otherwise all the decision-making is up to chef Arun.