  We study Ising Field Theory (the scaling limit of Ising model near the Curie
critical point) in pure imaginary external magnetic field. We put particular
emphasis on the detailed structure of the Yang-Lee edge singularity. While the
leading singular behavior is controlled by the Yang-Lee fixed point ($=$
minimal CFT ${\cal M}_{2/5}$), the fine structure of the subleading singular
terms is determined by the effective action which involves a tower of
irrelevant operators. We use numerical data obtained through the "Truncated
Free Fermion Space Approach" to estimate the couplings associated with two
least irrelevant operators. One is the operator $T{\bar T}$, and we use the
universal properties of the $T{\bar T}$ deformation to fix the contributions of
higher orders in the corresponding coupling parameter $\alpha$. Another
irrelevant operator we deal with is the descendant $L_{-4}{\bar L}_{-4}\phi$ of
the relevant primary $\phi$ in ${\cal M}_{2/5}$. The significance of this
operator is that it is the lowest dimension operator which breaks integrability
of the effective theory. We also establish analytic properties of the particle
mass $M$ ($=$ inverse correlation length) as the function of complex magnetic
field.
