The Supreme Court has upheld a case taken against the Mahon tribunal inquiry by Cork developer Owen O'Callaghan.
Last year Mr O'Callaghan successfully challenged the tribunal's refusal to allow his lawyers access to various statements made by builder Tom Gilmartin.
In the High Court, Mr Justice Iarfhlaith O'Neill declared that the tribunal's refusal to grant Mr O'Callaghan access to the statements breached his constitutional right to fair procedures and unreasonably hampered his right to cross-examine Mr Gilmartin.
The tribunal immediately appealed the decision, which could mean that many of its investigations would have to be reopened.
Today's Supreme Court ruling is likely to cause the planning tribunal significant delays and disruption.
Current hearings into a land deal at Coolamber in west Dublin have already been postponed until next month, pending today's ruling. More significantly, next week's planned start of hearings into bribery allegations surrounding the rezoning of Quarryvale could be affected.
Mr Gilmartin conceived the idea of developing a shopping centre at Quarryvale and was later joined in the venture by Mr O'Callaghan.
However, the two men parted company acrimoniously and Mr O'Callaghan eventually developed the Liffey Valley shopping centre in the mid-1990s.