Investigation launched into Mulhall knife photograph

The Irish Prison Service has launched an investigation after a picture emerged showing one of the Mulhall sisters joking with a knife inside Mountjoy Prison.

The photograph shows a smiling Charlotte Mulhall holding a large blade up to a male inmate's throat.

She has been moved to Limerick Prison following the emergence of the picture.

Charlotte and her sister Linda are both in prison over the killing of Farah Swaleh Noor - their mother's boyfriend - during a knife attack in March 2005.

Charlotte is serving life for murder and Linda 15 years for manslaughter.