The most likely future is as a predominantly rural analogue of the Scandinavian Centre parties, because while I don't see the urban vote returning any time soon, in a lot of rural areas they are the only organised or semi-credible alternative to Fine Gael at local level.
That is the one available niche I saw on the centre-right side. Ireland is significantly smaller than Sweden, Norway and Finland, but you have a strong rural/urban divide for such a small country.
They could hold the balance of power between FG and Labour/Sinn Fein.
Nordic centrists survive on a mix of support for rural subsidies/interests and some single issues to attract "non-socialist" voters who are not comfortable with the major centre-right parties: enviroment/ecology, euro-scepticism and a "humane" approach to immigration and crime. I suppose nationalism and perhaps euro-scepticism could still work as such profile issues for FF. Maybe also civil rights? (if FG has an authoritarian bias as you claimed).