Historians should have respect for facts - they shouldn't ignore inconvenient ones or deny truths. They are free to make their own judgements and arguments about why things happened or to re-examine motives and challenge conventional ideas, but they aren't free to invent history. You're allowed to make mistakes and misread things, but you aren't allowed to lie.
Once you do that, you're off the reservation, removed from the roll, no longer entitled to call yourself an historian. Irving claims to be self-taught and I'd say the quality of the teaching shows.
So whatever else Irving might be, he's not an historian.
Irving has for his own ideological reasons persistently and deliberately misrepresented and manipulated historical evidence; that for the same reasons he has portrayed Hitler in an unwarrantedly favourable light, principally in relation to his attitude towards and responsibility for the treatment of the Jews; that he is an active Holocaust denier; that he is anti-semitic and racist and that he associates with right wing extremists who promote neo-Nazism.Mr Justice Grey, summing up of Irving -v- Penguin Books & Lipstadt
Not one of [Irving’s] books, speeches or articles, not one paragraph, not one sentence in any of them, can be taken on trust as an accurate representation of its historical subject. All of them are completely worthless as history, because Irving cannot be trusted anywhere, in any of them, to give a reliable account of what he is talking or writing about. ... if wemean by historian someone who is concerned to discover the truth about the past, and to give as accurate a representation of it as possible, then Irving is not a historian.
Richard Evans, Professor of Modern History, Oxford University