Irreducible 4-manifolds with order two fundamental group and even intersection form

We construct smooth manifolds with order two ${\pi }_1$ and even intersection forms which are irreducible, meaning they do not decompose into non-trivial connected sums. Their intersection forms being even implies that their universal covers admit spin structures. Such manifolds are determined up to homeomorphism by their Euler characteristic $e$, signature $\sigma$, and whether they themselves are also spin. In the case that the manifold is spin, we construct irreducible manifolds for all but $17$ realizable coordinates in the region of the $(e,\sigma )$-plane with $c_1^2=2e+3\sigma \ge 0$ up to orientation. In the case that the manifold is non-spin, we construct irreducible manifolds for all but $24$ realizable coordinates in the region of the $(e,\sigma )$-plane with $\sigma \mathrm{/}8<-8$ and $c_1^2\mathrm{/}4>9$, again up to orientation. We construct these manifolds by taking equivariant fiber sums of Lefschetz fibrations and other symplectic manifolds which are simply-connected and spin. Along the way, we develop machinery to track when the spin structure is preserved during these operations.