Riley,
It's important to make the distinction that IRA support from the US came from Irish Republican Americans, not from the US people or its government.
In the case of Zimbabwe, for Western governments to intervene into a black African nation's domestic affairs presents serious political problems related to racism, primarily, but also colonialism.
The presence of oil has two sides. Oil revenues enable threatening behavior alongside the acknowledged need of the developed world for energy. Despotic oil regimes have been largely left alone to shackle their populations so long as they don't cause disruption or threaten other nations.
Do you promote more international meddling in the affairs of autocratically ruled nations, their oil resources notwithstanding?