The Radiological Protection Institute of Ireland (RPII) has called on British Nuclear Fuels Ltd (BNFL) to prove the plant's radioactive waste storage tanks could withstand a major terrorist attack.
The body said the recent increased risk of an attack on nuclear plants made it vital that BNFL made the disclosure.
In its annual report, the RPII also revealed that radioactive contamination from the Cumbrian plant found in seaweed was 150 times higher on the north east coast of Ireland than in seaweed on the south and west coasts.
It stated the amount found was small and did not pose a significant health threat.
The RPII said: "This report has gained heightened relevance from the fear, following September 11th, of a terrorist attack on Sellafield.
"The RPII considers that BNFL should now make available any evidence it can as to the capability of these tanks to withstand a major terrorist attack," it added.
The watchdog also questioned the fact that the consequences of an accident stemming from the storage of the liquid waste have not been properly assessed, putting Ireland at risk.
It said the discharge of radioactive waste from the Sellafield reprocessing plant is still "the dominant source of contamination in the Irish marine environment".
"Any contamination of the Irish Sea arising out of practises at Sellafield is highly objectionable from an Irish viewpoint," it said.
Despite the Institute's demands it stated it was safe to eat seafood landed at Irish ports and to enjoy fishing or swimming in the sea or to walk along the beaches on the east coast.
Sellafield has been a contentious issue between Ireland and the UK.
It was most recently raised by the Taoiseach with his British counterpart Mr Tony Blair at a meeting of the British-Irish Council at the end of last year.
PA