What about this quite simple solution?
import Data.Char (digitToInt)
int2intList :: Integral i => i -> [Int]
int2intList s = map digitToInt $ show s
main = print $ int2intList 12351234999123123123
gives [1,2,3,5,1,2,3,4,9,9,9,1,2,3,1,2,3,1,2,3]
This one is possible and a bit more universal, too:
int2intList :: (Read i, Integral i) => i -> [i]
int2intList s = map (read.(:[])) $ show s
main = print $ int2intList 12351234999123123123