Depending on the context it can be several words.
To illuminate a manuscript is maisigh le dathanna(decorate with colours).
To illuminate a room or a mind is soilsigh; tabhair solas do (give light to).
Cur soilse ar .. (To put light on buildings or streets.).
Maisigh le dathanna (to decorate with colours, as in a manuscript).
Tabhair léargas ar (To give enlightenment on a matter of learning).