Getting your knees done is a punishment for anti-social behaviour carried out by paramilitaries in their local area or jurisdiction . Normally several warnings, then a beating, then shot in the knees, usually for Joyriding, drug dealing, house breaking, robbery ect. It was meant as a deterrent, not to kill.
The fear was usually enough to make people leave N Ireland and head to the mainland where there could do what they like with only the law to deal with... or to Australia if you had managed to keep a clean record.