Many countries are concerned about visitors who come and who do not want to leave (or who cannot leave for some reason). To make sure that they can throw overstayers out again, they insist on passport validity beyond the intended period of travel. Three months or 90 days is common.
As an Irish citizen in the UK, you are not requesting permission to visit. For the time being, you are exercising your EU treaty rights to travel and work. The UK border officials are required to give you every opportunity to prove your citizenship. A passport is such a proof, up to the last day.