Both can be used as "도착하다" but they also can use as "reach to somewhere" or "reach to something" for example 그는 체력의 한계에 이르렀다(past verb of 이르다) = " "He faced his physical limit." 다다랐다.(Past verb of 다다르다) or 우리는 결론에 이르렀다.(Past verb of 이르다)= " We finally reached to the conclusion" 다다랐다(Past verb of 다다르다) They all normally used in written Korean. So in everyday Korea, I recommend you to use 도착하다 when you want to express "arrive" in Korean Cheers DK