what do they achieve that is differenet?
Insulation resistance is the physical resistance from the conductor to ground. It is a measure of the actual insulation properties. Dielectric strength is the maximum voltage a dielectric can withstand without rupturing. To determine each value, tests need to be performed. To determine insulation resistance, a test commonly referred to as a megger test is done. To determine dielectric strength, a dielectric proof test (hi-pot) is performed.
This has been discussed here before at length, do a search for the old discussion.