Irreducible complexity is meant to be a demonstration of the incorrectness of neo-darwinian accounts. There's nothing fallacious about such an argument. It's obviously a vicious circle to exclude any criticism of neo-darwinian accounts on the basis that it must be an 'of the gaps' argument and then conclude that there aren't any criticisms of neo-darinian accounts. The objection to make against irreducible complexity isn't that it's an 'of the gaps' argument (it's not; it's a perfectly valid was of objecting to neo-darwinian theory). The objection to make against irreducible complexity is rather that it happens to be wrong.