  We study the real-time dynamics of the order parameter $<\sigma(t)>$ in the
Ising field theory after a quench in the fermion mass, which corresponds to a
quench in the transverse field of the corresponding transverse field Ising
chain. We focus on quenches within the ordered phase. The long-time behaviour
is obtained analytically by a resummation of the leading divergent terms in a
form-factor expansion for $<\sigma(t)>$. Our main result is the development of
a method for treating divergences associated with working directly in the field
theory limit. We recover the scaling limit of the corresponding result by
Calabrese et al. [Phys. Rev. Lett. \textbf{106}, 227203 (2011)], which was
obtained for the lattice model. Our formalism generalizes to integrable quantum
quenches in other integrable models.
