What happens when an ionic compound is boiled?
Interesting question with no generic answer. Boiling the salts, or ionic compounds, makes them a plasma or freely moving ions is nothing but a misconception. The temperature is too low to convert a substance into a plasma. 


Let us start with ionic liquids which are room temperature cations and anions but made up of organic molecules. You heat them, to the state of boiling in the absence of oxygen. They decompose into various products. So nothing special, this is as expected for organic compounds.
Heating of non-decomposable salts at high temperatures, for example, NaCl, KCl: What will happen when we heat them to the point of boiling? Answer is nothing. They remain in vapor phase as NaCl "molecules". The vapors behave as gases. People have developed equations of state. Interestingly enough $\ce{Na2Cl2}$ (dimers) are found in gas phase.


See: Critical point and vapor pressure of ionic fluids including NaCl and KCl, Chemical Physics Letters, 105, 1984, 484-489


Ionic materials, KCl or NaCl, when introduced into flames or plasma convert into gas phase "atoms". For example $\ce{Na+}$ would be converted into a neutral Na atom. This is utilized in a completely different field in chemistry, called flame atomic absorption or flame atomic absorption spectroscopy.


In short, these complexities make science very interesting.