The key was made available on Thursday evening almost a week after the IT system was attacked.
The key was given to the Government by the organised crime group behind the cyber attack, but their reasons for doing so remain unclear.
Taoiseach Micheál Martin said: “No payment was made in relation to it at all. The security personnel don’t know the exact reason why the key was offered back.
“In terms of the operation of getting our services back and getting data systems back, it can help. But in itself, the process will still be slow.
He indicated the rebuilding process will be weeks rather than months.
Responding to reports that the criminals responsible intend to start selling and publishing HSE data online from Monday, Mr Martin said: “We’ve always said that the danger is there for data to be dumped.
“But the High Court action, an injunction that the HSE secured, is a very powerful and strong one, which makes it a criminal act to reveal any data that has been illegally obtained or has been stolen from the HSE system.” The main purpose of the injunction is to put internet companies such as Google and Twitter on notice of a legal prohibition on the sharing and publication of the information.
Mr Martin said: “We are very encouraged and appreciate the collaboration and co-operation from the major social media companies in respect of this entire attack.
“But also in terms of working with us to make sure that any data that is inadvertently put up will be taken down immediately.” He said paying the ransom demanded by the criminals – reported to be 20 million dollars, or 16.5 million euro – “would create a pattern of behaviour that would be damaging to the state into the future”.