Hello

I need some help:

How could I use dispel magic against fireball spell?  First of all, is an action spell…

I should use dispel magic against the creature that throws a fireball against me and my companions? 

In this case the effect of dispel magic deactivates the fireball or just its effect against me?

Or I should use dispel magic against de magical effect of the fireball spell?

Thanks

Rafael

No, Dispel Magic does nothing against instantaneous effects like this, you would want Counterspell. Dispel Magic is for spell effects with a duration, such as Hold Person, Scrying, or Symbol.

Generally, dispel magic ends spells that are actively affecting a player (so they have a duration longer than "instantaneous").  fireball is an instantaneous spell, so there would be no way to use Dispel magic to end the spell as even if readied, it would be cast after Fireball and after the fireball effect (the magic) was over.

counterspell will actually prevent the fireball spell, because its a reaction spell that triggers at the start of casting, not the spell going off.

Also, antimagic field, wall of force, and other spells like that could prevent you from being targeted by the spell, or cause the spell to fail before it could damage you.  These would need to be cast prior to the casting of fireball to work though.

There is some very generous interpretation of the spell where you could prepare Dispel Magic on your turn, maintain concentration on it and release it as a reaction to end a spell like fireball, but I'm confident that's 100% not how the spell is meant to work. Even if your DM is willing to take a very wild interpretation of the rules to allow this, it's still a hard gamble on your part... you have to willingly spend your whole turn casting and holding concentration on a 3rd level spell on the assumption that someone will be casting fireball, when you could just learn counterspell and avoid any confusion.

Quote from TransmorpherDDS >>

There is some very generous interpretation of the spell where you could prepare Dispel Magic on your turn, maintain concentration on it and release it as a reaction to end a spell like fireball, but I'm confident that's 100% not how the spell is meant to work. Even if your DM is willing to take a very wild interpretation of the rules to allow this, it's still a hard gamble on your part... you have to willingly spend your whole turn casting and holding concentration on a 3rd level spell on the assumption that someone will be casting fireball, when you could just learn counterspell and avoid any confusion.

Yea that's not allowed by RAW and it certainly isn't RAI (the SAC has a section about it). But if a DM wants to make it a rule then ofc they can, I would suggest not though.

Counterspell stops a Fireball.

Dispel Magic is how you negate a Delayed Blast Fireball after it's been cast, but before it explodes.

The Sage Advice Compendium explains in details why instantaneous spells cannot be dispelled;

Can you use dispel magic on the creations of a spell like animate dead or affect those creations with antimagic field? Whenever you wonder whether a spell’s effects can be dispelled or suspended, you need to answer one question: is the spell’s duration instantaneous? If the answer is yes, there is nothing to dispel or suspend. Here’s why: the effects of an instantaneous spell are brought into being by magic, but the effects aren’t sustained by magic (see PH, 203). The magic flares for a split second and then vanishes. For example, the instantaneous spell animate dead harnesses magical energy to turn a corpse or a pile of bones into an undead creature. That necromantic magic is present for an instant and is then gone. The resulting undead now exists without the magic’s help. Casting dispel magic on the creature can’t end its mockery of life, and the undead can wander into an antimagic field with no adverse effect. Another example: cure wounds instantaneously restores hit points to a creature. Because the spell’s duration is instantaneous, the restoration can’t be later dispelled. And you don’t suddenly lose hit points if you step into an antimagic field! In contrast, a spell like conjure woodland beings has a non-instantaneous duration, which means its creations can be ended by dispel magic and they temporarily disappear within an antimagic field.