Basically, in order to be considered pretty by Koreans, you have to fulfill their five standards of beauty. These five standards are slim figure, light and bright skin, V line face, small lips, and larger eyes plus double eyelids (Korean beauty standards – Wikipedia). There’s many more but these five are the most basic ones. The reason why Irene is considered really pretty and even top class visual is because she fulfills those standards without any flaws in regard to her physical appearance.
If you look at each and every picture of Irene, you could identify those five features. Her figure is very slim, she has a very light and bright skin, V line face, small lips, and double eyelids. Beauty standards vary by each country and each region. However, in East Asia, the standards are almost the same. Korean beauty standards are also almost similar with both Japanese and Chinese beauty standards which explains why Irene insanely popular in East Asia.