Because he's hoping he won't have to. It may just be possible for FG to form a government with a number of Independents.
In many ways that might end up better for the country long-term. We'd actually end up with a sensible ideological split to the country, with a rightwing government and a reasonably large leftwing opposition. No more splits based upon a 90 year old treaty.
It would be shit for the next few years, but they will be shit anyway, and by the next election we would get a governement made up of a leftwing coalition. If FG and Lab go in together this time round there is a very big chance that 5 years from now FF will regain an awful lot of lost ground and everything will go back to how it's been since the inception of the state.