* Geal: This is the most common and generally translates to "bright" in terms of light or color. Think "a bright light" (soilse geal) or "a bright red" (dearg geal).
* Solais: This means "light" but can imply brightness. You might use this in a sentence like "a bright day" (lá solais).
* Gile: This is the noun form of "brightness" or "radiance." You'd use it more descriptively, such as "The brightness of the sun" (Gile na gréine).
* Solasach: This is an adjectival form meaning "bright" or "illuminated."
The best word to use depends heavily on what you're describing as "bright." If you can provide the sentence you want to translate, I can give you a more accurate translation.