Martin McGuinness claims a united Ireland is inevitable. Is he right?

'A united Ireland is inevitable' - according to Sinn Fein's Martin McGuinness. The Deputy First Minister said that he is working to bring it about but won't put a date on it. He added "it will happen when it happens" in an interview with Liam Clarke in yesterday's Belfast Telegraph.

Big words from the Deputy First Minister - he says a united Ireland is "inevitable". Is it? How far away is it if it is? Or is it just wishful thinking?