Recent research has shed some new light on this question. A dissertation that came out of Rutgers in 2007 called the whole idea of ius sacrum into question.
Johnson takes a look at the evidence behind the standard interpretation. For reference, for years everyone relied on Adolf Berger's article in the RE and, later, his entry in Encyclopedia Dictionary of Roman Law, which I produce below:
Ius Sacrum. Strictly connected with ius divinum and ius pontificum. It embraces the legal principles and institutions which are connected with the relations of men to gods, with questions of cult, sacrifices, temples, consecrations, graves, and sacerdotal functions, whenever they may occur. The jurists Servius Sulpicius and Trebatius wrote on the subject of the ius sacrum. In oldest times the ius sacrum exercised a considerable influence on private law, the knowledge of legal rules and their interpretation and applicability having been a monopoloy of the priests.
Problem is, the evidence behind such a reading is flimsy. I noticed this too when looking for evidence for the phrase in PHI:
Naturae iura sacra sunt etiam apud piratas.
The sacred laws of nature are also [found] among pirates.
There are so few references, that something like this thrown in is a dead giveaway that something is amiss. The ius sacrum here isn't a legal code, but simply a description of ius, it's really ius naturae, quod sacrum etiam est.
Johnson goes through the rest of the passages and cannot find either a legal or literary precedent for it. Instead, he thinks scholars have been conflating the terms ius pontificium and ius sacrorum, but there too are differences.
The ius sacrorum (attested as early as Cicero, e.g. De Domo Sup 41.1), or, perhaps, more correctly, the iura sacrorum, was a body of religious law that governed a family's sacra, holy objects, masks, penates, etc. These were the items that Romans used in conjunction with offerings to the gods, establishing a relationship between the domus and the divine. Unless I misunderstand the argument, the ius sacrorum that deal with public sacra would fall under the domain of the pontifical college, and thus part of the ius pontificium.
This latter ius is the broader one, and is encompassed by various sub-iura (e.g. ius caerimoniarum; see p. 112 for his reasons for concluding thus).
All of this so far is a type of ius gentium, man-made law that governs individual nations. The Roman implementation of the ius sacrorum or ius caerimoniarum would not necessary apply to the Greeks, Carthaginians, or Gauls. This can be contrasted though with the ius naturale, 'natural law', those laws (sacra in Seneca's opinion) which the gods gave mortals.
This is where fas and nefas come in. You don't need to make a specific law or listen to the advice of the pontifices to know that cannibalism is wrong. It's not just wrong on a technical or legal level, but morally wrong (cf. Ov. Met. 6.585). It's a broach of human nature, and the gods will not only punish you (which can happen from carelessness), but sentence you to an eternity of torture in the underworld.
As such, nefas is not legalistic, but rather laws were created to address issues of fas and nefas, not that they necessarily needed to do so.
