I'm reading Introduction to Statistical Learning, currently Chapter 2 about the Bias-Variance trade-off.
In all examples the irreducible error is 1, i.e. $Var(\epsilon) = 1$. I read in Why is the variance of the error term always 1 in examples of the bias-variance tradeoff? that this is likely for pedagogical reasons.
My question is, how would you actually go about to find the irreducible error? Is this even possible?