Prediction: The feds will ignore any defiance.
The feds could get in a snit for the defiance of a county sheriff and cause him problems but the local sheriff could make life a living hell on any federal employee regardless of agency or department if the feds got in a snit about their authority being questioned by some lowly sheriff who is actually elected by the people.
No back up for any fed raids and actually being present to represent the citizens.
Any fed employee for any agency could get pulled over and ticketed for traffic infractions, they could have their cars torn apart continually if the sheriffs drug dog “hits” on the fed car.
Depends on how far the sheriff wants to push the issue but they could make life miserable on federal employees and their families.