I wonder whether it has maybe occured to you that an early medieval language/culture like Old Irish does not have the same concept of words like "theory" as we have today.
I suppose "dliged" would work (= law, principle, etc.), so you could probably say "dliged cach ní". Why you would want to have that as an Ogam tattoo is beyond me, though.
Writing poetry in: Scottish Gaelic, German, English.
Reading poetry in: Latin, Old Irish, French, Ancient Greek, Old Norse.
Talking to people in the shop in: Lithuanian, Norwegian, Irish Gaelic, Saami.
Listening to people talking in the shop in: Icelandic, Greenlandic, Finnish.