The practical politics are that because of the swing to SF, FF and FG stand back and let SF try to form a government without them. This can't work. See my post here.
In any subsequent negotiations between FF and SF and anyone else, the nominee for Taoiseach will be in the mix and subject to agreement. I suspect that SF will be willing to forgo the office of Taoiseach in order to get more of their policies into the programme for government. Micheál Martin is desperate to not be the only FF leader never to have been Taoiseach, SF can use this to their advantage.