The term 'considerable irritant' can be summed up by the crossword solution 'THORN'. A THORN is a sharp, pointed object that typically grows on plants and can cause irritation when it comes into contact with skin. Thorns are often used as a defense mechanism by plants to protect themselves from predators. They can also be used to deter animals from eating or trampling on the plant. In addition, thorns can be used to make clothing and other items more durable.