After sitting dry for so long there is no guarantee it will even hold liquid. It doesn't matter how microbe-free it might be if it won't hold your beer. The only way to test the barrel is to fill it with water, let it soak and then see if it leaks.
It is very likely to have an intolerable colony of acetobacter and wild yeast. I wouldn't try using it for anything other than trying to make vinegar.