It is considered that the reason for reading '達' ('dachi') as 'daru' is that the nissho (one of the four tones in Chinese) 't' of kango (words of Chinese origin) which was used in the Middle Ages changed into the liquid 'l' in the Korean pronunciation of kanji (based on adopted Chinese pronunciations), and 達 'dat' changed into the sound 'dal' in the Korean Peninsula on its way to Japan.
「達（ダチ）」を「ダル」と読むのは中古漢語の入声tが朝鮮語漢字音で流音lに変化したため達datは朝鮮半島でdalに変わり、その音が日本に伝わったためとされる。