To "radiate" is "to send out in every direction." Usually it means light or energy, but not necessarily. The spokes of a wheel radiate from the hub. To "irradiate" is "to bathe in radiation." The sun radiates light. The grass is irradiated by sunlight. In the scientific field of radiometry, the unit of "radiance" is "watts per steradian per square metre," and the unit of "irradiance" is "watts per square metre."