Just as the header asks, what levels of irreparable ecological and environmental damage was caused by the war?
When I read about the island invasions of the Pacific War such as Peleliu, Okinawa and the many other islands that saw the war, and the preceding naval bombardments that typically accompanied them and then the intense battles that would follow; I think of all of the native, tropical species caught up in that fire storm and how it affected those biomes and ecosystems.
The same relative question applies to the war in both Eastern and Western Europe and how those battles and cataclysmic conflicts affected the land in an agricultural as well as ecological sense.
One ecological impact from my area of expertise—WW2 U-boats—is that explosions scared away many of the fish off the US East Coast in 1942. The elasticity of water means that explosive energy travels farther and with more force than explosions in air, so blasts can kill or injure sea life within a wide radius. And there were a lot of explosions in US waters during 1942. Each German torpedo carried a warhead of ~500 lbs of explosives, and each of the (many) depth charges employed carried between 150-600 lbs of explosives.
A United Press International news release dated 7 March 1942 mentioned the effect on fish: "According to Bryan Travis of Cape Charles, VA, a chief boatswain's mate in the Coast Guard who also held many saltwater fishing records: 'game fish off the Atlantic Coast have mostly disappeared and won’t be back until the war ends. Torpedo and shellfire explosions have run the fish away…they can sense danger, locate food, and plot the future. When the first torpedo exploded off the coast it probably killed every living thing within a quarter mile and a like distance in depth. It may be years before another fish will enter that area'."^(1)
Whales were also attacked in error by US ships & aircraft because they resembled U-boats both visually and on sonar.^(2)
Additionally, the many tankers torpedoed in US waters spilled large quantities of oil—although I’m not aware of the specific ecological impact at that time, nor the degree to which this has been studied. Many tankers sank with oil still in their cargo holds. Oil that washed up on New Jersey beaches in 1967 prompted a Coast Guard reassessment in which divers examined several major tanker wrecks off the Northeast. Those wrecks ruled out as threats included the Gulftrade, sunk on 10 March 1942 (which I have dived.)^(3)
In 2015, satellite photos spotted oil slicks over the wreck of the Coimbra—the second ship sunk in US waters in 1942—south of Long Island. This led to a successful 2019 salvage operation that pumped out the remaining 476,000 gallons of oil from the Coimbra's tanks. The effort required 193 dives and was one of the largest of its kind ever conducted in US waters.^(4)
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More can always be said, but this older answer might be of interest for you.