The Clarence Consortium, which includes Bono and The Edge, was given the go ahead to demolish six protected buildings on Wellington Quay to make way for the revamp.
Conservation officer with the IGS, Emmeline Henderson, told the Herald a judicial review "is certainly something we would consider if there is recourse".
However, she pointed out the move would have to be made on the basis of a technicality in the Bord Pleanala ruling -- the decision itself cannot be reviewed.
The U2 band members, along with property developers Paddy McKillen and Derek Quinlan, were given the green light to press ahead with the transformation of the 44-bedroom boutique hotel into a 141-bedroom, five-star hotel and spa, complete with restaurant, bar and fresh food market.
But the controversial plan has alarmed conservationists as it involves a massive reconstruction of the Clarence, an art deco building dating from 1937, four Georgian buildings from the early 19th century and Dollard House, which was built in 1886.
All are listed buildings and only the facades along Wellington Quay in the south inner city will be preserved.
The plans were designed by internationally renowned architect Norman Foster and include provision for a roof-top, flying saucer-like structure which will dominate the quayside landscape.
The board made its decision by a seven-to-one majority against a recommendation by senior planning inspector Kevin Moore that permission be refused.
Mr Moore characterised the scheme as "conceptually brilliant but contextually illiterate".
But the board said the project would "provide a building of unique quality and architectural distinction".
It also ruled that "the continued historic hotel use" was sufficient to constitute the "exceptional circumstances" required in legislation to permit the demolition of protected structures.
"In deciding not to accept the inspector's recommendation to refuse permission, which involves the part-demolition of protected structures, is permissible because of the exceptional quality of the design," the board's report said.
It added that the new design would not undermine the integrity of the Liffey Quays Conservation Area.
The massive revamp was granted permission by Dublin City Council in November but that decision was appealed.