What is the relation between ionization energy and electropositivity?
Electropositivity is rarely used by itself. It is much more commonly seen as just the opposite of electronegativity. While electronegative is more or less defined to the point where numeric values can be found for each element in a given system (sometimes excluding noble gases), electropositivity has no such scale I am aware of.

Ionisation energy or potential are clearly defined energies that can be experimentally directly determined and are tabulated across physical chemistry textbooks with little to no variance (the main variance being experimental error).

The two terms are hardly related; neither by definition/physical meaning nor by value trend.