The phrase "irradiance increase" means that the amount of light or energy radiating or shining in a certain area has become greater or has intensified. Full definition
[Response: You don't expect it to be completely the same since there are differences: GHGs cause stratospheric cooling, solar irradiance increases cause warming there — GHGs have a very even effect across latitudes, solar is stronger in the tropics. (realclimate.org)
Estimated increases since 1675 are 0.7 %, 0.2 % and 0.07 % in broad ultraviolet, visible / near infrared and infrared spectral bands, with a total irradiance increase of 0.2 %. (junksciencearchive.com)
Therefore, the overall irradiance increases in correspondence with higher numbers of sunspots. (scientificamerican.com)