  We use heat capacity measurements as a function of field rotation to identify
the nodal gap structure of CeIrIn5 at pressures to 2.05 GPa, deep inside its
superconducting dome. A four-fold oscillation in the heat capacity at 0.3 K is
observed for all pressures but with its sign reversed between 1.50 and 0.90
GPa. On the basis of recent theoretical models for the field-angle dependent
specific heat, all data, including the sign reversal, imply a d{x^2-y^2} order
parameter with nodes along [110], which constrains theoretical models of the
pairing mechanism in CeIrIn5.
