  With the discovery of the electromagnetic counterpart of the gravitational
wave source GW170817 the multi-messenger era is started. The identification of
an electromagnetic counterpart is crucial to understand the nature of the
detected gravitational wave sources and to maximize the scientific return of
their detections. The role of the instrument THESEUS/IRT will be crucial in
this field, in particular in localizing afterglows of gamma-ray bursts within
few minutes from the trigger and in identifying optical/NIR isotropic emissions
such as kilonovae.
