Emails between senior managers at the ESB seen by RTÉ's This Week programme have raised what Sinn Féin's Mary-Lou McDonald has described as "serious concerns".
In 2010, unions and the ESB concluded an agreement to address a €2bn deficit it the company pension scheme.
In the annual report for that and all years following this agreement, ESB changed the accounting treatment of its pension scheme from a defined benefit-type scheme to a defined contribution-type scheme.
This removed pension liabilities from ESB's balance sheet.
The move also significantly improved both the company's ability to borrow money, moving the company, in its own words, from "sub-investment grade" to "solid stand alone".
According to ESB, the company borrowed €3.1bn at rates ranging from 2.5% to 6.25% after the change in accounting.
Had it not achieved the change, the company estimates it would have been facing a pension deficit in accounting terms of €3bn.
However, according to an email between two of ESB's most senior managers in July 2011, ESB's auditors KPMG had been prepared for this change in accounting treatment of the company's pension scheme.
It was changed on the basis that the Group of Unions within were in agreement with the management on the matter.
Emails seen by RTÉ's This Week programme show managers at the highest level in the ESB were aware that the unions "were always at odds with what was stated in the accounts".
In one email from July 15 2011, a senior manager wrote: "What we needed in order to get the change (to the accounting treatment) across the line was some evidence to show KPMG that the GOU were not completely at odds with Management on the understanding behind the Pension Agreement and so they would not publicly comment that they disagreed with what was stated in the Accounts in relation to the change in accounting.
"As you know, the GOU were always at odds with what stated in the Accounts in prior years as we always said the Scheme was not a typical balance of costs scheme."
Another email on the same date referred to a request by ESB's auditors for the company to approach the Group of Unions for a clear statement that they agreed the Management position, but the company "refused to countenance this".
The manager went on to write: "Should Brendan (Brendan Ogle - Secretary of the ESB GOU) decide he does not agree with the wording and send something to us in writing, it would unavoidably force us to change the accounting treatment and all that entails."
Written objections by the trade union representatives were put on file with the ESB within days.
On 27 July 2011, at a meeting between the ESB and the GOU, the minutes recorded in full a statement about the annual report read by Brendan Ogle. They said: "The Pension Agreement did not change how deficits in the scheme are resolved in any way.
"Whatever the ESB finance director said to financiers and auditors in relation to the scheme is between the company and the financiers and auditors".
A further letter from the Group of Unions in October 2012 in the aftermath of the Government pension levy outlined the GOU's belief that the ESB continued to have liabilities to fund the pension scheme.
Change is 'very worrying for workers'
Speaking on RTÉ's This Week programme Ms McDonald said the emails are significant as they establish a number of "very worrying things".
The Deputy Sinn Féin Leader said it seems ESB management went ahead with the accountancy treatment of the pension fund.
She said this was despite not having agreement from the workers or the unions and despite KPMG raising some concerns and looking for assurances there had been some level of agreement.
She said that it is "very worrying for workers".
Ms McDonald said Minister for Communications, Energy and Natural Resources Pat Rabbitte has questions to answer as the State still holds a 95% stake in the ESB.
She said she "cannot understand how this happened, this shift on the balance sheet, having those accounts signed-off by the Department, by the Minister - perhaps by Cabinet - and an alarm bell not to have gone off in Minister Rabbitte's mind".
The ESB has refused to comment on whether it brought these documents to the attention of its auditors.
The company continues to account for its pension scheme as a defined contribution scheme.
In answer to a query to Mr Ogle by This Week as to whether there had been any direct contact between the GOU and KPMG about this matter, the programme received the following statement:
"I acknowledge your call yesterday and your question as to whether the Group of Unions had alerted KPMG as to our view that ESB had a liability for pension scheme deficits and current funding issues.
"Firstly let me state that KPMG never contacted us in this regard.
"Further, I can confirm that the Group of Unions did not contact KPMG.
"We viewed this as unnecessary as we were aware that ESB had an obligation to disclose any Group of Unions demand regarding pension funding the further(to the 2010) pension agreement to KPMG.
"I confirm that I was personally made aware of this obligation by ESB on Thursday 11 October 2012 when I was shown a letter from KPMG to ESB setting out the obligation of ESB to disclose any Union demands regarding pension funding."
ESB has declined to comment on the matter.
A spokesman for Mr Rabbitte said the minister was unavailable for comment due to a legal action being mounted by four ESB pension scheme members against the ESB.
A spokesman for KPMG declined to make any comment.
However, the ESB did issue a statement in repsonse to the This Week programme.
It said: "Four claimants, funded by the ESB Group of Unions, have issued High Court proceedings against ESB in relation to the ESB Pension Scheme and the manner in which ESB accounts for the Scheme in its financial statements.
"ESB will fully defend those proceedings and will respond at the appropriate time and in the appropriate forum to the claims made.
"ESB wishes to state that its dealings with its auditors have always been and continue to be fully truthful and accurate, and is fully satisfied that its financial statements have been prepared in accordance with all relevant international accounting standards and laws.
"Given that these matters are now before the courts, it does not intend to comment further."