"Pax deōrum" is the harmony between the divine and the human, achieved through correct religious practice.

The grammar in this song is strange. In the first line, domine is the vocative singular of dominus "lord", but venīte is the (second person) plural imperative of veniō "to come" (whereas the singular imperative is venī).

The official lyrics gives the beginning of the song as one unbroken line "domineoveniteosacramentum(discesm)eodeo", also suggesting that it should be interpreted as "sacrāmentum discēs meō deō". That's also a bit strange, because discēs is the second person future indicative of discō "to learn": "sacrament, you will learn from my God". I believe the line as sung has eō deō in the ablative "with/from/by that God", and I'm unsure what is meant by it.

The section in Irish is the same as in the song Athair ar Neamh, and the bridge is a quote from Horace's Epistles.

References:
https://www.pathname.com/enya/tmot.html
http://enya.sk/music/the-memory-of-trees/pax-deorum/
https://en.wikipedia.org/wiki/Glossary_of_ancient_Roman_religion#pax_deorum