Not sure about Byrne, have always considered him an insufferable dose, but he has 4 things going for him.
He has made a huge contribution to Irish life.
He isn't a politician
He isn't any of the other candidates
He will probably only do 1 term.
Its not like the office requires a politician, and it won't effect the running of the state if he dies in office. Therefore I don't see his old age as a drawback, a positive in fact because we will be able to replace him with someone better in 7 years.
And if we can start a trend of the office going to someone who has contributed to civic life, rather than a politician, it opens the way for someone else like that next time round. The likes of Niall Quinn or Seamus Heaney could do a fine job in Aras.