Enterprise Energy Ireland (EEI) has accused SIPTU of using out-of-date information to make claims of "massive" gas reserves off the Irish west coast.
It has also criticised comments made on the gas issue by the independent election candidate for Mayo, Dr Jerry Cowley.
The company was responding to last week's call by SIPTU's offshore committee on opposition political parties to spell out their position on offshore oil and gas policy in the light of the valuation of the Corrib gas field.
The SIPTU committee chairman, Mr Padhraig Campbell, quoted a report by international oil and gas consultants, Wood MacKenzie, from 1998
This report put the size of the Corrib field off Mayo at seven trillion cubic feet of gas. It said this was estimated to be worth €21 billion.
The Mayo GP, Dr Jerry Cowley, has already called for a renegotiation of the State's deal with EEI on the Corrib field, following its multibillion-euro takeover by Royal Dutch Shell.
Dr Cowley described the current deal as a disgrace and said the £4.3 billion valuation placed on Enterprise Oil by Royal Dutch Shell indicated that the Corrib field project had been undervalued.
A spokesman for EEI told The Irish Times that SIPTU was referring to an out-of-date study done in 1998 by Wood MacKenzie.
"The most up-to-date Wood MacKenzie study is February 2002, where the field reserve is estimated at 0.875 trillion cubic feet of gas, and not the seven trillion as seemingly suggested by SIPTU" he said.
"While there has been much commentary in the lead-up to the election about the suggested 'massive' reserves of gas 'allegedly' awaiting recovery off the west coast of Ireland, sadly to date these so-called 'massive' reserves have not yet been discovered, despite major investment in exploration over the past 30 years," the spokesman added.
Referring to Dr Cowley's comments on the Shell takeover, he said the £4.3 billion valuation placed on Enterprise Oil by Royal Dutch Shell was based on "an extensive portfolio of production and exploration activities including those in Brazil, the North Sea, Italy and the USA.
"The Corrib field represents a relatively small part of the Enterprise Oil portfolio, and there is no justification for Dr Jerry Cowley to contend that the valuation of the company is an indication of an undervaluation of the Corrib gas field," the spokesman said.
Two months ago a Mayo-based oil and gas exploration expert, Mr Mike Cunningham, said that Irish people were being kept in the dark about gas reserves off the coastline.
He added that information on these were relayed to Government officials as far back as early 1998.
He referred to a written submission by Mr John Downey of EEI to the Bord Pleanála oral hearing last March on the proposed gas terminal, pointing out that this said Corrib was a moderate-sized gas field, but also referred to prospects identified in the Slyne-Erris basin.
Mr Downey's report said it was highly probable that any further gas discoveries in the Slyne-Erris basin, where Corrib is located, would be produced via the proposed Bellanaboy Bridge Terminal.
This would extend that terminal's life by 10 years, from 2018 to 2028.