The package Miscellaneous`WorldData` is used by the package Miscellaneous`WorldPlot` and is automatically loaded when you load Miscellaneous`WorldPlot`. Most users do not need to be aware of it, but if you want to expand on the list of countries WorldPlot knows about you will want to update Miscellaneous`WorldData` as well as Miscellaneous`WorldNames`.
WorldPlot works by creating a Mathematica graphics object that represents the map. Each country on the map is given as a Polygon primitive. To make a map from a list of countries, Mathematica must know the vertices of these polygons for each country in your list. These data are contained in the package Miscellaneous`WorldData`. As noted previously, the vertex data are given as ordered pairs of latitude and longitude expressed in minutes. By convention, latitudes north of the equator are positive while those to the south are negative, and longitudes east of Greenwich are positive and those to the west are negative.
The vertex data for a country is attached to WorldData of the name of that country. The data are in the form of a list of lists of points. This is so multiple polygons can be specified for each country.
You can specify your own database of geographic coordinates by setting the WorldPlot options WorldDatabase and WorldCountries. The setting for WorldDatabase is the name of the function that is applied to a country name to yield its polygonal vertices. It defaults to WorldData. WorldCountries is set to a list of accepted country names, and defaults to World (from the Miscellaneous`WorldNames` package). Miscellaneous`WorldData` contains all of the WorldData["countryname"] rules that WorldPlot has access to.
This loads the package.
Here is the list of the vertices of the polygon that give the map of Ireland.