Irradiance is the area density of flux arriving at a surface (E/A); radiant exitance (M) is the area density of flux leaving a surface. The units are W/m2. More generally, when the flux distribution is not constant, irradiance at a point is defined as: E = dΦ/dA. When imagining the derivative, it is important to not think of positional change, but growing/shrinking area and the effects on the flux. If you are imagining the possibility of a negative flux, you are probably thinking about position instead of area change.
For a sphere with a point light source at the centre, the irradiance is given by E = Φ/4πr2