Thornir Alekeg
Albatross!
The way I see it, since it states it counterspells a spell "cast upon he wearer" it would not be effective against a fireball. Now if I were to house rule this to allow the ring to be effective against aoe spells, here's how I would justify it:
A fireball can be counterspelled by another fireball spell. If the counterspell had to be placed precisely on the point of emanation of the cast fireball, that would be virtually impossible - like trying to knock an arrow out of the air with another arrow, therefore the counterspell has to only be within the area of effect of the spell and it cancels the spell like two offset waves. Based upon this, the ring of counterspelling would magically detect that it was within the area of effect of a fireball spell and would immediately react and perform its counterspell.
A fireball can be counterspelled by another fireball spell. If the counterspell had to be placed precisely on the point of emanation of the cast fireball, that would be virtually impossible - like trying to knock an arrow out of the air with another arrow, therefore the counterspell has to only be within the area of effect of the spell and it cancels the spell like two offset waves. Based upon this, the ring of counterspelling would magically detect that it was within the area of effect of a fireball spell and would immediately react and perform its counterspell.