This has always been a painful area in philology for several reasons:
- In many cases, verses in the original text can have several inferred meanings. For some languages and cultures, this is actually part of the tradition. For example, in Literary Chinese, each verse of the Tao Te Ching can be translated in several ways, each with a significantly different meaning.
- Translations, especially early translations, were usually done by missionaries and are coloured by the religious convictions of the translator. For example, in early translations of Chinese text, you ill often come across the phrase "Mandate Of Heaven", where the word choice of "heaven" is preferred to match Christian cosmology. But this is not exactly the same or limited to what the Chinese text infers.
- Idioms can be very hard or impossible to translate, as this study details.
If you have to work from translations, I would recommend to compare as many translations as possible and pay special attention to textual variants. Specifically for the Táin Bó Cúailnge, I would recommend ignoring the Victorian translations as they tend to censor the explicit content of the original text.
The version by (Kinsella 1969) is generally considered to be the most acurate translation in English. If you master German, (Windisch 1905) is considered to be complete, and lacks alterations and omissions due to conflicts of interests in the mind of contemporary Irish scholars.
I would also recommend verifying against decent studies that were already done on the subject matter, such as A Study Guide for Anonymous's "Tain Bo Cuailnge"
Hope this helps.