The process involved pouring hot pitch, or tar (mainly used at the time for lighting purposes), into a conical shaped paper "cap", which was forced onto a bound suspect's head and then allowed to cool. Less elaborate versions included smearing a cloth or piece of paper with pitch and pressing onto the head of the intended victim. The "pitchcap" was then torn off, taking lumps of skin and flesh with it which usually left the victim disfigured for life.
The torture was usually preceded by the crude shearing of the victim's hair, and many accounts report that ears were often partly or fully severed during the cutting. Refinements to the torture included unbinding the victim's feet to allow the spectacle of them running about in agony and in some cases, deliberately smashing their own heads in an attempt to end the torment. Another variation involved adding turpentine or gunpowder to the "pitchcap" when cooled then setting it alight.
The torture was probably devised as a response to the short "cropped" hairstyle popular in Ireland at the time (hence the nickname "croppy" given to Irish rebels), which was inspired by the French Revolutionary style, a repudiation of the long hair and wigs of the aristocracy.
Pitch has long, even in antiquity, been used (like other hot liquids, even melted metal) to pour into a victim's orifices. However, both those techniques were usually faster and often lethal, so less suitable as torture proper, rather as capital punishment.