To have been beaten without mercy into a state of retardation.

The 'Mong' part is derived from Mongoloid or Mongolism and refers to general retardation and the second part comes from the verb 'Fong' meaning to beat mercilessly.
Television report on a tragic accident:

"The police officer on the motorbike swerved to avoid a pedestrian who stepped onto the road, he then struck the side of a bus before cascading into a lamppost causing a severe and permanant brain injury."

Carlow youth:

"Yea he was mongfongled!!!"
by Micko91 December 13, 2006
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