Irritation also has non-clinical usages referring to bothersome physical or psychological pain or discomfort.
In humans, it is a mild form of suffering, often with anger about the suffering; in particular, if applicable, anger at the person who caused it. This can also be oneself, e.g. when forgetting something or doing something one deems to be stupid.
It is not proven that oysters can feel pain, but it is known that they react to irritation. When an irritating object becomes trapped within an oyster's shell, it deposits layers of calcium carbonate (CaCO3), slowly increasing in size and producing a pearl. This serves no purpose to the oyster, pearls do not attract mates for the oyster or perform any other function. It seems impossible to find an evolutionary advantage for the ability to produce the pearl, thus it can be explained only as a reaction to an irritation.
It has also been observed that an amoeba avoids being prodded with a pin, but there is not enough evidence to suggest how much it feels this. Irritation is apparently the only universal sense shared by even single-celled creatures.
It is postulated that most such beings also feel pain, but this is a projection - empathy. Some philosophers, notably René Descartes, denied it entirely, even for such higher mammals as dogs or primates like monkeys - Descartes considered intelligence a pre-requisite for the feeling of pain.