Longan fruits (Dragon Eye and Euphoria Longan have both been used to refer to Longan fruits) have some traditional usage for alleviating cognitive losses and improving memory. Limited evidence so far suggests this may be true, as oral administration of the fruits to rats appears to exhibit potent neuroprotective effects and has twice been implicated in enhancing cognition.
However, the composition of Longan fruits currently is not remarkable. It is a great source of ellagic and gallic acids, which are potent anti-oxidants; these compounds are not inherently linked to improvements in cognition nor are the Procyanidins or catechins found in Longan. There actually does not appear to be a large amount of compounds known to be unique to Longan fruits, which may be discovered in the future. No bioactive is linked to the cognitive protective or memory enhancing properties.
Beyond that, the polysaccharides appear to be able to interact with the immune system and can act in an immunostimulatory manner. At least one study suggests that the potency is comparable to Ganoderic Acid (from Reishi) but this needs to be replicated; it is another promising avenue.
Other interactions of Longan such as interactions with fat mass, skin quality, or the colorectal cancer interactions are currently unremarkable. There is not enough evidence to suggest Longan fruit as a recommended option for anything although it might be a useful functional food to ingest for cognitive purposes.