I'm looking for this old law text the Senchas Mar from the eighth century. I can find a lot of things talking about it, but nowhere can I find the actual text of the law itself. Can anyone help me track it down, hopefully in English translation?
Basic information: https://en.wikipedia.org/wiki/Early_Irish_law#Senchas_M%C3%A1r
Well, this is a relatively easy one to answer! Though, I apologize that it isn't about to be a very useful one.
The Senchas Már does not survive to us modernly in a single form. As I am sure you have read while looking into it, it was a collection of laws compiled by a law-school (or schools) in an area controlled by the Northern Féni (deduced through names referenced in the material and, very interestingly through some scholarship regarding where specific sorts of trees grow).
We can tell that, by the Old Irish period, a specific 'set order' had been developed for the texts in the Senchas Már, and scholars have spent the last century or so discussing and deducing which various legal texts that survive to us modernly were once part of the Senchas Már.
So, what has happened is that, while the Senchas Már existed once upon a time, we have no single manuscript that contains the whole thing. We have many different legal texts in different manuscripts, some of which we have (pretty convincingly) argued were originally part of the Senchas Már tradition.
This means, unfortunately, the reason you can't find the actual text of the Senchas Már is because it no longer exists. And, while we do have several different texts that were part of it (Bretha Crólige, Bretha Déin Chécht, Catshlechta, Bechbretha, etc) translated, the majority of the material is yet to be translated (and yet to be edited even!).
If you have any specific questions regarding what part of the Senchas Már you want to look at, I might be able to help direct you to a translation. However, if you are just interested in medieval Irish law in general, I would suggest A Guide to Early Irish Law by Fergus Kelly. I have provided the citations for some of the other component legal texts I referenced above in my bibliography.
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Sources
Kelly, Fergus. A Guide to Early Irish Law (Dublin: Dublin Institute for Advanced Studies, 1991).
Charles-Edwards, T.M. and Fergus Kelly, Berchbretha: an Old Irish Law-Tract on Bee-Keeping (Dublin: Dublin Institute for Advanced Studies, 1983).
Murray, Kevin. *'*Caitslechta and Other Medieval Legal Material Relating to Cats,' Celtica 25 (2007): 143-159.
Binchy, D. A. 'Bretha Déin Chécht,' Éiru 20 (1966): 1-66.
Binchy, D. A. 'Bretha Crólige,' Éiru 12 (1938): 1-77.