The young female bird had been followed by thousands of people online as part of a satellite tracking project run by the NPWS and local community group IRD Duhallow.
It is an offence under the Wildlife Act to kill a hen harrier, as the birds are listed as a protected species, with penalties including significant fines and/or imprisonment. Local Gardaí have been informed of the incident.
Dr. Barry O'Donoghue of the NPWS, who oversaw the satellite tracking project, said: "Through the satellite tracking programme, thousands of people were given an insight into the life of this bird and her progress was followed from Kerry to the Wicklow Mountains. The tracking system showed that the bird visited Meath, Louth, Monaghan and Armagh and rested by the shores of Lough Neagh, before making a long distance journey all the way to the Atlantic cliffs of South County Cork.
"The bird stayed there for most of her first winter with a number of other harriers. From these older birds, she would have learned of good hunting places and safe places to spend each night. In late 2014 she returned to South Kerry, back to the very site where she was born. It seems likely that she might have returned to breed there this summer, but unfortunately her life has been cut short.
"The deliberate killing of a protected species, such as the hen harrier, is a serious offence under the Wildlife Act. The NPWS has launched an investigation into the unlawful killing of the bird, and local Gardaí have been informed."