Sinn Fein should nominate Norris
I see the Red C/Paddy Power opinion poll shows Senator David Norris to be the most popular choice for president ahead of Mairead McGuinness, Bertie Ahern, Fergus Finlay and Michael D Higgins.
His difficulty is going to be in securing a constitutional nomination to run in the election, as this requires the support of 20 members of the Oireachtas, or four county or city councils. Up until now, this has meant that only Fianna Fail, Fine Gael and (sometimes) Labour have had the wherewithal to nominate a candidate, and they generally use this opportunity to nominate one of their own.
However, Mary Robinson was technically an independent when nominated by Labour as she had resigned from the party over the Anglo-Irish Agreement and so there is a precedent for a party nominating a non-member.