This is just a wild stab in the dark, but perhaps the tree seemed somewhat 'glad' because it was deranged by anger and grief over the destruction of its kin. The only reason I offer this extrapolation is because I've seen how some people react when someone close to them has been seriously injured or killed. They get this mad, sadistical happiness that makes them tempt fate, sort of like the tree playing with fire. I suppose it's something psychological, but I'm not an expert in that area at all.
Peace
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'Dulaman na Binne Bui, Dulaman Gaelach/ Dulaman na farraige, 's e b'fhearr a bhi in Eirinn!'
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