Dispatching on only Int is too restrictive. Generic code is normally preferable, so it’s better to define special behavior only for the exceptional cases you want to rule out (Bool input, in this case).
But if you don’t want to write the extra methods to manage those exceptions, at least I recommend you to allow any other Integer that is not Bool. Bool happens to be the only subtype of Integer which is not either Signed or Unsigned, so you could write:
getty(m, index_offset::Union{Signed, Unsigned}=0) = m[:, index_offset+2]