From the
Incorporeal Subtype:
Even when hit by spells or magic weapons, it has a 50% chance to ignore any damage from a corporeal source (except for positive energy, negative energy, force effects such as magic missile, or attacks made with ghost touch weapons).As a Fireball is none of the listed exceptions, it would be subject to the 50% chance not to cause damage.
Whether the damage is caused by a regular attack, touch attack or a failed save (or a successful save in the case of partial effects) doesn't matter, still subject to the 50% unless it falls under one of the exceptions above, or the ability description itself makes a specific exception.
