Micheal Martin has rejected coalition with SF on the basis of the party's paramilitary past. There is no escaping that.
Leo has rejected coalition with SF on the basis of their rejection of the special criminal court and the unacceptability of their economic policies. Plenty of wriggle room there.
SF could conceivably cannibalise FF votes, many FF people feel comfortable with SF.
No FG voter is ever going to vote for SF.
It may sound odd to say this in the 21st century, but my neighbour, a FF voter, will never vote for FG because they (the free state government) murdered his grandfather. And can you blame him. He and his extended family and his friends and neighbours amount to a sizeable number of votes. A FF/SF government could easily see him becoming a SF voter after the next election.
There are also people whose grandfathers were killed by anti-treaty forces, they will never vote SF.
All this makes SF less of a threat to FG in terms of stealing voters.
A FG/SF collation is not that far fetched.