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Politicians in Northern Ireland have been told that President Obama will not cross the border when he visits the Republic in May.

The president will be prevented from doing so by longstanding diplomatic constraints that make crossing the border into Northern Ireland impossible unless he first travels to London.

The Irish government is already feverishly preparing for the historic May visit, which will see the president return to his family's historic roots in Moneygall, County Offaly.

Irish Taoiseach (Prime Minister) Enda Kenny confirmed this week that president Obama cannot cross the border without first setting foot in Britain.

"The problem actually is that the president, under existing protocol, he's not allowed to go to Northern Ireland without first having to go to Britain," Kenny was quoted as saying in the Newtownabbey Times.

"So if President Obama were to decide to go close to the border, actually from a protocol perspective he's expected to go to London before he would go to Northern Ireland."

The specific start and end dates of President Obama's trip have not yet been determined, but it's understood the president wants to arrive in the Republic before a planned state visit to Britain on May 24.

Referring to formal requests to visit the north by Northern Ireland's First Minister Peter Robinson and Deputy First Minister Martin McGuinness, Kenny replied that the issue was out of his hands.

"But I'm very glad he's coming here and I hope that the people of Ireland will give him a brilliant and wonderful welcome as he comes to associate himself with part of his ancestry."