I am not sure if the 774 event had an impact upon global climate in that year, and I dont think that it is likely that it influenced teh climate two decades later. Certainly, tree ring data suggests that 774 was a cold year, but one needs to be careful in assuming a causal link.
As to what caused famines and crop failures two decades later, it is difficult to say. There does seem to be a large eruption around AD 799/800 as evidenced in tree rings and ice cores, which I believe to be the basis for the period of darkness after the death of Constantine. Writing around AD 810-15 the chronicler Theophanes the Confessor records, after the capture and blinding of Byzantine emperor Constantine VI in August AD 797, that:
“The sun was darkened for seventeen days and did not emit its rays so that ships
lost course and drifted about. Everyone acknowledged that the sun withheld its rays
because the emperor had been blinded” (Mango and Scott, 1997)
Such an event is suggestive of a volcanic dust veil or ash cloud observed from Constantinople, possibly from a Mediterranean eruption. . Note that we are told Constantine’s blinding occurred after August 797, but we cannot know how long after. It is conceivable that the obscuration event could have been as much as a few years after the Constantine’s capture and blinding, but that the two events were associated as direct cause and effect by the popular psyche at the time, and recorded as such. It is thus tempting to link this event with the sudden cooling observed in Swedish pine that occurred in AD 800 (see graph ), and which is also evident in reduced Irish oak growth.