Keynesianism, in a sentence, is the economic philosophy that in a recession, taxes should be cut and spending risen, while in times of growth taxes should be risen and spending cut- this has the effect of reducing the effect of recessions while slowing economic growth at boom times, basically making the economy "flatter" as opposed to a cycle of huge booms followed by deep depressions. Governments who have subscribed to it have often failed to implement Keynesian policy during times of boom (which would be high taxes and low government spending, resulting in large surpluses), resulting in high inflation- as we saw in the 1970s.

This is not a failure of Keynesianism as much as it was a failure of governments to implement the correct policy.

It certainly is not socialism, or social-democracy, or an endorsement of high taxes as a way of making people's lives better. Of course low taxes are nice, but government services are needed. If medieval governments had taxed their poor AND their wealthy and distributed some of their wealthy's income to the poor as welfare, then the poor would have been far better off than had the government let them farm their tiny patch of land without any taxes.

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If you remove the English army tomorrow and hoist the green flag over Dublin Castle, unless you set about the organisation of the Socialist Republic your efforts would be in vain.[...]

It would be tantamount to a public declaration that our oppressors had so far succeeded in inoculating us with their perverted conceptions of justice and morality that we had finally decided to accept those conceptions as our own, and no longer needed an alien army to force them upon us.