This word USED to mean an Irish traveler and typical scum from Ireland, nowadays it is used as a term to refer to people with a thick dublin accent, wears Burberry, Adidas, Nike, Reebok and other typical sports clothing.
It has become VERY common with youths and the thing is is that older adults still feel it means the same as what it used to. Well, it doesn't.
This word has become as popular as the english term chav, which would probably be a better choice to search for what a knacker means.
That girl is such a knacker, she keeps saying "Howya" and has an Adidas jumper on :O