Annabeth wrote:
It was
"Bhí mé i mo luí, liom fhéin."
And the translation read, "I was in my bed, alone."
So I got confused

I wouldn't say that the translator was wrong...in fact, that's a good, idiomatic, translation. Translating it literally as "I was lying down, by myself" wouldn't convey the same sense as the Irish, so the translator was correct to phrase it in that way.
That said, if this was in a text book, or a book written for learners, the translator probably should have included somewhere the information that "bhí mé i mo luí" is an Irish idiom -- another way of saying "I was sleeping."
Redwolf