The US ambassador to Ireland, Mr Richard J.Egan, is understood to be resigning from his post because of his frustration over the limits of his job.
Mr Egan unexpectedly submitted his resignation after just 15 months in the post a week ago.
He will continue to serve into the new year, with his date of departure determined by President Bush.
Mr Roger Marino, who co-founded the EMC Corporation, a data-storage company, with Mr Egan in 1979, said the ambassador told him yesterday he felt frustrated at the many administrative duties he faced, while being left out of important discussions about peace and jobs in the region.
"He felt he was doing a lot of things that weren't as productive as they could've been, and felt a little frustrated," Mr Marino told the Boston Globe newspaper.
"His capabilities to do good are endless, and yet he was in a non-productive role."
A US State Department official said Mr Egan's resignation was not requested by President George Bush, but that it would be accepted.
Additional reporting by AP