I don't know that this particular one is used, but analagous to "Dar Dia!" you could say "Dar an talamh, an mhuir is an spéir!", but that's an exclamation. I guess you could say "Geallaim duit ar an talamh, an mhuir is an spéir." to make a promise on these. There's another word, "dearbhaigh" which is used for asserting that something is true on official forms, but I'm not sure that this is used in other kinds of oaths.
I suppose in my own case "Dar Déithe" would be more appropriate then the singular form of God.
Does anyone have any thoughts or suggestions? Has anyone run into this line in the manuscrpits?