The Irish President Mary McAleese was heard exclaiming 'wow' as the Queen spoke the opening words of her speech at a state banquet in Gaelic.
Speaking in Dublin castle, the monarch said: "A hUachtarain agus a chairde (President and friends)", earning prolonged applause.
And afterwards experts declared her pronounciation "excellent".
Commentators said the gesture demonstrated Britain's respect for an independent Ireland and will help exorcise ghosts of the two nations' bloody past.
The Queen also offered her "sincere thoughts and deep sympathy" to the victims of Ireland and the UK's troubled relations.
Also joining her at the gala in the same dining room where Queen Victoria was received was Prime Minister David Cameron.
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