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Webster 1913 Edition

adv.

In an irreversible manner.

adv.

In a manner which precludes a reversal or repeal.

irreversibly ‎(comparative more irreversibly, superlative most irreversibly)

irreversibly in The Century Dictionary, The Century Co., New York, 1911
irreversibly in Webster’s Revised Unabridged Dictionary, G. & C. Merriam, 1913