What does the Gaelic phrase An di allaigh an di aigh An di allaigh an di ne ullah An di ullah be nith rah Cair di na ulla nith rah Cair feal ti theo nith rah An di allaigh an di aigh mean?

While you're correct that the passage comes from author Cate Tiernan's Sweep series in book #3 entitled, "Blood Witch" on page 46, the Words are a mix of traditional Irish Gaelic & Scottish-Gaelic. You can't just plug the phrase in and get something because the passage is cobbled together. I had to do 1 Word or so at a time, and found that by adding Words as they appear in the sentences, some of their overall meanings changed completely. So, below is what I got, as well as a few possible overall translations of the aforementioned passage. I spent a few hours doing this translation, and although it got a little bit frustrating at times because there was NEVER a clear-cut translation of the passage as I had hoped there might be, or at least a one-and-done translation that made sense, it was interesting (and sometimes quite laughable) with what some of the Words wound up translating to on their own, or with the other Words surrounding it in the sentences they were in, as well as the meanings of the Words in Irish (traditional Gaelic) versus Scottish-Gaelic. You'll see what I mean...

So, here goes:

[Note: For easiness of understanding and to save some time, Words translated from Irish (traditional Gaelic) will have an (I) next to it and those translated from Scottish-Gaelic will have an (S) next to it just to make stuff easier if you feel like looking stuff up for yourself or just for reference of originating language.]

allaigh = God (S) an di allaigh = The Goddess (S) an di allaigh = is she alone (I)

an di aigh = the two cows (S) an di aigh = is she here (I)

an di allaigh an di aigh = the house or the house; a house or a house (S)

an di allaigh an di aigh = is she alone is she here (I)

"An di allaigh an di aigh an di allaigh an di ne ullah" (Is she a god or is she a woman? Is it god or is it ullah?) (S)

an di ne ullah = the god or ullah (S) (Note: "ullah" spelled with the "h" at the end had NO TRANSLATION according to my various searches; however, the Word "ulla" without the "h" on the end meant "ready" when translated into Scottish-Gaelic, and "Apples" in Irish-Gaelic.)

cair = love (S) cair = friend (I)

cair feal ti = love you (S) cair feal ti = love her (I)

cair di na = love her (S) cair di = love her (I) cair di na = love her na (I) (Note: The Word "na" did not translate in Irish-Gaelic.)

cair di na ulla = love her (S) (Note: The Word "ulla" doesn't seem to affect this translation from the the main phrase that came up BEFORE adding the Word "ulla" to it)

cair di na ulla nith rah = love her more than anything else (S)

cair feal = love meat (I) feal = meat (I) feal = veal (S) (as in baby cow meat) theo = hot cair feal ti theo = hot meat (S) cair feal ti theo = love you hot meat (S) (Note: if the Words are done Word-by-Word, or in small sections, it comes out as this, which sounds like a person's love letter, or ode to cooked meats, LOL