Separating something from usual judgement and guaranteeing its protection, especially the funds of a project.
This term has been in use since the 1980s to denote the funds that are set aside for a project and cannot be spent on anything else. Before that it had a more general meaning, of anything that was protected; for example, this comment from Roger Eliot Fry, in his Letters, from January 1903:
"B.B. [Bernard Berenson] should not have it said that he is capable of political scheming to ring-fence Italian art."
These figurative uses all of course derive from the literal 'ring-fences' which were used to confine stock on farms.