The American Ambassador to Ireland, Richard Egan has tendered his resignation to President Bush, after just 15 months in the post.
A spokeswoman for Mr Egan, gave no reason for the decision, but she he will continue to serve into the new year.
She added that Mr Egan, a wealthy businessman, considered it a very high honour to serve as ambassador to Ireland.
Ambassadors ambassadors serve at the pleasure of the president, and according to protocol, Mr Egan will have to wait until his resignation has been accepted.
Mr Egan was one of the founders of the computer data storage company EMC Corporation.
