Side-by-side Latin and English here.
It's not really a "poem," but a kind of "song" called plain chant. The music used in daily Catholic worship and rites was standardized for the entire liturgical year and a system was formalized for writing it all down. Pope Gregory the Great is generally credited for doing this about thirteen hundred years ago, but it's really a lot more complicated than that. The melodies and songs themselves are probably much older.
Dies Irae painted by Federico Correa.
