は・
は・ならいざしらず sets A aside as a possible exception—“A may be another matter”—and then makes a firm contrasting judgment about B. The real point of the sentence is always the judgment about B.
How the contrast works
A is mentioned only to be detached from the present judgment: perhaps A would be understandable, acceptable, or simply different. B is the actual case under discussion, and the speaker firmly says that B cannot be treated like A. Common contrasts involve different people, time periods, places, degrees, and conditions.
The two main shapes
Notice that each sentence quickly moves past A and places the important evaluation in the second half.
百年前はいざしらず、現代の企業が紙の記録だけに頼るのは危険だ。
A hundred years ago may have been another matter, but it is dangerous for a modern company to rely only on paper records.
AnalyzeThe earlier period is set aside; the warning about modern companies is the main assertion.
一度の入力ミスならいざしらず、同じ誤りが毎日続くのは看過できない。
A single data-entry error might be another matter, but the same mistake occurring every day cannot be overlooked.
Analyze本当に予算が足りないならいざしらず、確認もせずに計画を中止するのは早計だ。
If the budget were genuinely insufficient, that might be another matter, but canceling the plan without even checking is premature.
AnalyzeThe full clause before the pattern supplies the hypothetical exception.
Choosing among nearby expressions
ならともかく is the more neutral, conversational choice when A can simply be allowed as another matter. ならまだしも specifically presents A as at least tolerable before criticizing a worse B. はともかく can merely postpone discussion of A and does not require the sharp exceptional contrast found here.
More examples
昔はいざしらず、今では海外との会議もオンラインで開ける。
The past may have been another matter, but now even meetings with people overseas can be held online.
Analyze休日ならいざしらず、平日の昼間に三時間も店を閉めるわけにはいかない。
A holiday might be another matter, but we cannot close the store for three hours in the middle of a weekday.
Analyze本人が望んだならいざしらず、家族が勝手に進路を決めるべきではない。
If that were what the person wanted, it might be another matter, but the family should not decide their future path on its own.
Analyze緊急時ならいざしらず、通常の手続きを省略する理由はない。
An emergency might be another matter, but there is no reason to skip the normal procedure.
Analyze二、三分遅れるならいざしらず、連絡もなく一時間待たせるのは失礼だ。
Being two or three minutes late might be another matter, but making someone wait an hour without contacting them is rude.
Analyze十人程度の組織ならいざしらず、社員千人のこの会社を一人で管理するのは不可能だ。
An organization of around ten people might be another matter, but it is impossible for one person to manage this company of a thousand employees.
Analyze