Int affects Mana Consumption and Casting Time

Fireball's base use is 100 total *(20 per charge) so the mana use after fire mastery is 80 (100 * .80). When calculating net mana cost, you must factor in mana regeneration, which is base +0.05 per second (Night time bonus is 300%, placing it at +0.15 per second). 20s*0.05mp/s gives +1 mp, resulting in the net mana cost per time being reduced by 1 mana, etc.

Fireball only costs 25 MP per charge if you are a Giant according to the wiki's current Fireball page.

To date, every percentage based system in Mabinogi to date has utilized a compounding formula system, so it is not unreasonable to conjecture that the int bonus would function the same way. Regardless of which way it functions, one would notice a significant drop in MP usage, which could easily be determined to be either additive or compounding in nature, should INT have such an effect; so the exact numbers for the purposes of testing the theory of "Does INT decrease mana use?" is irrelevant.

EDIT: I used 15% for the mana usage reduction from fire mastery instead of 20%. My bad. I've corrected it in the above post.