Unusually High BLEU score on a NMT model
According to recent publications, it is not impossible to get BLEU scores as high as yours for English→Irish. Nevertheless, without any other knowledge, they certainly seem too high.

From the command line arguments, there does not seem to be any evident problem.

The most probable explanation is, as you already pointed out, a data leakage between validation/test and training. Note that, while you removed exact duplicates, you may be getting partial matches that go unnoticed. You may want to look into different similarity metrics. The most straightforward is the Jaccard Similarity.