The absence of snakes in Ireland gave rise to the legend that they had all been banished by St. Patrick, He chased them into the sea after they attacked him during a 40-day fast he was undertaking on top of Mt Croagh Patrick.
The problem is all evidence suggests that post-glacial Ireland never had snakes. So maybe the snakes that Patrick expelled were not real terrestrial snakes but symbolic celestial snakes?
In my post "Fulacht fiadh - salt extraction facility?" I talked about the climate change patterns in Ireland over last 5,000 years and how they could have affected people's ability to extract salt from sea water.
The Greenland Ice Cores provide a temperature record for the last 5,000 years. Clearly manifest are three temperature peaks which correspond with the archaeologically and historically documented Warm Periods in the North Atlantic region: Minoan Warm Period 1450–1300 BC, a Roman Warm Period 250 BC – 0 AD, the Medieval Warm Period 800–1100 AD. On the chart you can also clearly see the well documented extreme cold period known as the little Ice Age 1350 to 1850 AD.
"The Bronze Age Optimum" starts with the sudden sharp rise in temperature during the Minoan Warm Period which started right about 1500 BC. How warm was Atlantic northern Europe during the Minoan Warm Period can be discerned from the fact that during the Minoan warm period, millet was grown in southern Scandinavia. Today Millet is grown in tropical and subtropical regions, it is an important crop in Asia, Africa and in the southern U.S.. The average annual temperature in Mississippi and Alabama where millet is grown today is about 10 degrees, which should be compared with today's average annual temperature in Denmark, which is 8 degrees.
The temperature after the Minoan Warm Period drops and has another minimum around 1200 BC rising to another maximum around 1000 BC. After that it oscillates around relatively stable low value until it suddenly starts to rise around 250 BC. This is the beginning of the Roman Warm Period
The Roman warm period started quite suddenly around 250 BC. Some studies in a bog in Penido Vello in Spain have shown that in Roman times it was around 2-2.5 degrees warmer than in the present. The Roman warm period is amply documented by numerous analyses of sediments, tree rings, ice cores and pollen – especially from the northern hemisphere. Studies from China, North America, Venezuela, South Africa, Iceland, Greenland and the Sargasso Sea have all demonstrated the Roman Warm Period. Additionally, it has been documented by ancient authors and historical events.
How warm was Northern Europe during the Roman Warm Period can be seen by the fact that during the culmination of the Roman warm period olive trees grew in the Rhine Valley in Germany. Citrus trees and grapes were cultivated in England as far north as near Hadrian’s Wall near Newcastle.
The temperature then has a sudden drop during the first century AD but it then rises as suddenly and stays stable high until the end of the fourth century AD when it suddenly drops during the first half of the fifth century to an extreme low level.
The dates of St Patrick's life are uncertain. His own writings provide no evidence for any dating more precise than the 5th century generally. The Irish annals for the fifth century date Patrick's arrival in Ireland at 432 AD. His sermon on the Mt Croagh Patrick, during which he banished snakes from Ireland must have happened soon afterwords...
St Patrick's arrival to Ireland coincides with the beginning of the sudden huge drop in average temperature, which during his life fell to the level comparable to the temperature during the so called "Little ice age".
So Patrick arrives to Ireland. He defeats the old Sun God Crom Dubh, whose holy mountain was the same Mt Croagh Patrick from which Patrick drove snakes into the sea. He converts people to Christianity. And at the same time during the destruction of the old religion based on sun worship, the sun "dies". The heat of the sun disappears.
In my post "Three suns" I talked about the symbolic link between Snakes and Dragons and the heat of the sun. This link was clearly preserved in Slavic mythology. Snakes come out during the hottest part of the year and thus symbolize the summer. Slavs believed that snakes "suck the heat out of the sun" and that this is why summer sun eventually looses it's heat and autumn and winter arrive. The dragon is actually the symbol of the summer sun's extreme heat, destructive heat which brings drought.
The belief that snakes and dragons were driven out of Ireland by Patrick, could be remnant of the blame that the Sun worshiping Pre-Christian Irish put on Patrick and his Christianization efforts for the sudden (and probably catastrophic) climate change. Basically they blamed Patrick for driving the summer out of Ireland.