Much of what I tell you is no doubt quite irrelevant, but still I feel that it is best that I should let you have all the facts and leave you to select for yourself those which will be of most service to you in helping you to your conclusions.
There are no more uses of "irrelevant" in the book.
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The sentence should be deleted because it introduces information that is irrelevant to the passage.
The writer should deleted the sentence because it provides background information that is irrelevant to the paragraph.