Great to discuss, this more common than I realized.
I don’t know the specific etymology, or it matters. Sampling some native responses from Kansai-Chubu-Kanto region, they will hear and use it. But I asked a native from Tohoku, they would not use it…but then someone from Kyushu may use it as well (!). So seems everyone know what it is though but I believe いらない and 分からない are still considered ‘standard’ but this appears too common not to learn.
If curious, here were a few native responses in regards to いらん and わからん. For simplicity of a non-native asking, they say Kansai-ben but still used in various places.
Then interestingly, another native saying it is or less used but think it is Nara-ben (いらん) which I guess is just overall Kansai-ben. There is a whole thread of responses from different areas saying they use it.