Sometimes monetary policy and federalism can interact in interesting ways. To grasp a particular relation, such as that of Romania in the European Union, it is first necessary to keep in mind that monetary policy is not federalism and vice versa. An anti-federalist, for example, might have an incentive to conflate the two concepts out of a desire to deny the existence of a federal system already underway.
The complete essay is at "Is the E.U. a Federal System?"