If damage dealt by a source you control is being divided among multiple permanents and/or players while you have delirium, that damage is divided before doubling.

The damage is dealt by the same source as the original source of damage. The doubled damage isn't dealt by The Rollercrusher Ride unless it was the original source of damage.

If another effect (or effects) modifies how much damage a source you control would deal while you have delirium—by preventing some of it, for example—the player being dealt damage or the controller of the permanent being dealt damage chooses the order in which any such effects apply. If all of the damage is prevented, the effect of The Rollercrusher Ride's first ability no longer applies.