What the roles will actually be, is decided in practice. People in decades-old democracies may forget that, nevertheless, even in old democracies, current practice can change the constitutional roles -- see the change in the relative weight of the US Presidency from McKinley (rise of the imperial President with foreign policy dominance) and FDR (dominance in interior policy too), or Bliar's change of the weight of the PM, or indeed as you mention in your diary, Mary Robinson's re-definition of the Irish Presidency.

_{*Lunatic*, n.
One whose delusions are out of fashion.}