The first part of the book provides a comprehensive history of Oriental scholarship the West, starting in Ancient Greece. Irwin tells us that, unlike what Said is saying, not all scholarship was done with an imperial political agenda in mind. Irwin rightly claims that Said only discusses French and British Orientalism. In doing so, he forgets to mention Oriental scholarship from the Soviet Union and Russia, Nazi Germany, Germany in general (esp. 19c Oriental philology), medieval Europe etc. This historical overview gives nuance to Said's understanding of Oriental scholarship, namely that all of it is interwoven with an imperial and colonialist political agenda. I agree with Irwin that Said's Orientalism is too often unchallenged in Western universities, so this historical overview is important.
However, Said never wanted to give this type of historical overview. Early in his book he even said that he is aware of other types of Orientalism than the French and British ones, such as Dutch Orientalism, and he invited other scholars to look into it. Rather, Said, drawing from Foucault, wanted to give a genealogy (which is *not* a historical overview) of Orientalism. Through Foucaldian discourse analysis, Said wanted to show that the West has defined itself and functioned in history as colonialist. I'd argue that Said's book is theoretically inconsistent (something he later admitted himself, saying he "designed it that way"). Unfortunately, Irwin only devotes three pages to Said's use of Foucault and Said other main influencer, Gramsci. He makes the interesting point that Gramsci's and Foucault's ideas are incompatible and that Said did not realise this. Nonetheless, his critique falls short and he missed a chance of properly criticising Said's nihilism that is the result of Foucault's understanding of representation (=if all representations are constructs of power, then that means that no representation can accurately describe external reality, because truth itself is a representation). Irwin probably thought that the theoretical underpinnings were secondary to Said, and that therefore he could "destroy" Said's Orientalism with just historical facts, but as I just showed, he is wrong. Even if he did not think this, Irwin's methodology is flawed because he did not consider the question of whether historical (empirical) facts can demolish a (non-empirical) theory. As a student of philosophy myself, I am inclined to say this is not possible or at least, this is a complex problem that Irwin (and historians in general..) should have considered. But then again, Irwin would tell me that I am overstating Said's emphasis on theory, and perhaps he is right.
Lastly, Irwin forgets to mention the fact that in 2001, Said distanced himself from his earlier nihilism by saying that not all Oriental scholarship is bad, as long as one is "methodologically conscious" of what they are doing.