Why can't we say taitnionn e? (I don't have accents)
And how do we know whether a word is male or female?
I think you use é with nouns or adjectives, and the word is (remember "is cailín í?"). If you were saying it is shining, maybe you'd use é, but taitníonn is a verb. That's my guess, based on what we've learned at least!
As for gender of a word, it seems to have to do with declension. I've gotten a bit frustrated with it myself, so I found this link, and maybe it'll help. http://nualeargais.ie/gnag/subst3.htm
If the object that is shining is masculine. If it's feminine, use sí. However, though, the two big things I think of as "shining" (moon and sun) are both feminine, so to say "It (the sun) shines", you would say Taitníonn sí.