The High Court in Dublin has ruled that the Irish government has "unreasonably delayed" holding a by-election in Donegal.
The decision came following a legal challenge by Sinn Fein senator Pearse Doherty.
The seat in Donegal South-West has been vacant since June last year.
The court ruled that the delay was "unprecedented" and it amounted to a breach of Senator Doherty's constitutional rights.
Mr Justice Nicholas Kearns said it was the longest delay in the history of the Irish state and represented a significant proportion of the five-year parliamentary term.
He said the relevant legislation did not set out the time period within which an election should be called but added that he was satisfied it should be interpreted as requiring it to be called within a reasonable time.
He said he was not going to order the government to call the by-election or not to oppose a motion to call it.
But, he added that he hoped any clarification provided by his judgement would have that effect.
Sinn Fein member of parliament Aengus O'Snodaigh said his party would introduce a writ to have the election in the Dail (Irish parliament) on Thursday.
Speaking after the ruling, Senator Doherty said the taoiseach (prime minister) should call an election.
"That the government chose to waste taxpayers' money fighting this case is a disgrace," he said.
"Today's court decision has struck a fatal blow to the life of this government."
Speaking in the Dail, the taoiseach said the government would be considering the judgement and its implications but said further comments at this time on the matter would be premature.
The vacant Donegal South-East seat is one of four vacancies in the Dail.
The ruling coalition of Fianna Fail and the Greens has only a wafer-thin parliamentary majority with 82 members supporting the government and 79 in opposition.