What is the difference between "Past cost" & "Sunk cost" ? Which is most likely to be "Irrelevant cost" in decision making?

The major difference is that Sunk Costs are100% loss, means there shall be no recovery.

Why you are saying that these are not relevant, indirectly these are relevant for further planning 

sunk cork is nearly salvage cost

A past cost is an out of use price so it is an irrelevant cost 

The amount paid to purchase something is a sunk cost