The electricians' union the TEEU has said the decision does not overturn existing pay rates and conditions.
However the electrical contractors group, the NECI, said the judgment means all current agreements "are cancelled".
The five-judge Supreme Court ruled the Industrial Relations Act of 1946, which made provision for the agreements, raised serious issues of incompatibility with the Constitution.
There was a "wholesale grant, indeed abdication of law making power to private persons unidentified and unidentifiable at the time of grant to make law in respect of a broad and important area of human activity and subject only to a limited power of veto by a subordinate body", the judgment said.
The court said the act allowed the parties to an employment agreement "to make any law they wish in relation to employment so long as the Labour Court considers them to be substantially representative of workers and employers in the sector".
Eamon Devoy of the TEEU said certain employers were looking for a loophole in the law and they had found one today.
Speaking on RTÉ's News At One, Mr Devoy said that while the ruling meant that REAs were no longer constitutional, other contracts of employment could not be unilaterally altered without agreement.
Mr Devoy said that contractual rights and conditions could only be altered by the agreement of the parties involved and that as a result any terms agreed between employers and workers were not affected.
However, he said the removal of the minimum pay rates set by REAs would allow contractors to undercut the current rates paid to technical workers covered by the 70 REAs registered with the Labour Court.
He said this would leave the sector exposed to workers from overseas being hired by contractors at a rate no greater than the current minimum wage of €8.65 per hour, considerably less than the rate paid to many technical workers under the REA agreements.
Also speaking on News At One, Dave Butler of the National Electrical Contractors of Ireland welcomed the decision.
He said that all REAs are now unconstitutional and invalid.
Mr Butler said that up to today the NECI had been bound by REAs which it had no part in negotiating.
He said that REAs had been agreed by the larger employers and the unions, and the NECI "had been left outside the door".
Now that these REAs have been declared unconstitutional, Mr Butler said that its contractors intend to grow their business, and employ people in what he called "a realistic way".
However, he denied that this would lead to a race to the bottom, and said NECI arrangements would be "far from the minimum wage".
A statement from Minister for Jobs Richard Bruton said the Government intends to study the finding and get legal advice before commenting in detail.
He said: “The judgment has the effect of striking down Registered Employment Agreements put in place under the 1946 Industrial Relations Act.”
He said the agreements which set pay and conditions for workers in five sectors including electrical contracting and construction are affected by today's judgment.
Existing contractual rights of workers in sectors covered by Registered Employment Agreement are unaffected by today's ruling, he added.