"immutable" = "cannot be changed"
"interventions" = various therapies, drugs, tutoring, etc.
"irrespective" = (in this case) "regardless" or "despite"
It doesn't matter what treatments are tried (or when), someone with a learning disability will always have that learning disability.
Note that this doesn't mean interventions are pointless. Most learning disabilities are of the form "it is much more difficult to learn something", and not of the form "it is impossible to learn something". Teaching someone to learn to read and write may take longer, but is still worthwhile.