History of Ireland
In 1857, he published History of Ireland, by Geoffrey Keating, D. D., translated from the Original Gaelic, and Copiously Annotated (New York, 1857). Dr. Todd, in his preface to the Wars of the Gaedhill with the Gaell, says: "His translation of Keating is a great improvement upon the ignorant and dishonest one published by Mr. Dermod O'Connor more than a century ago, but has been taken from a very imperfect text, and has evidently been executed, as he himself confesses, in great haste." O'Mahony's notes are copied from O'Donovan's Four Masters, and it was on this ground that Hodges & Smith procured an injunction against the sale of the book in the United Kingdom. The mental strain to which O'Mahony was subjected in the preparation of this work, which brought him no pecuniary gain, affected his reason, and he was removed by his friends for a short time to a lunatic asylum.
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