The Irish Airline Pilots' Association, which represents holders of 7% of Aer Lingus shares, says a takeover of the airline would "not be good for our members".
Evan Cullen, head of the association, told the Oireachtas Transport and Communications Committee today that a takeover would result in "incredible gains for Aer Lingus executives".
Mr Cullen also questioned whether IAG had the cash for the takeover and suggested the UK company may have to sell or mortgage assets to raise the money.
"Aer Lingus does not need IAG - IAG needs Aer Lingus," he stated.
He also said that catering and maintenance of flights which is currently carried out in Dublin could be transferred abroad.
Both IAG and Aer Lingus currently have a base in Belfast, he said, adding that it was inconceivable that they would continue with two pilot bases there.
If IAG were to sell the Heathrow slots in the future, he said, whoever took them over would be obliged to operate them on the same routes for a period of possibly three to five years, but after that they would be "free to put them where they like".
SIPTU's Owen Reidy said Aer Lingus workers had engaged in restructuring which allowed the company grow profits and give safety and security to staff and employers.
He said a takeover should be blocked if there are no tangible guarantees on connectivity, the Heathrow slots and transatlantic routes.
Mr Reidy also said that the union regretted the fact that the previous Government took the decision to privatise Aer Lingus.
Matt Staunton, from the IMPACT trade union, said the bonus culture at Aer Lingus was linked to the company's share price and led to executives encouraging takeover offers for the airline.
The chambers of commerce in Dublin, Cork and Shannon also attended the Leinster House committee meeting.
IDA Ireland, which had been due to attend, will now write to the committee chair to outline its views on aviation policy.
Committee Chairman John O'Mahony said: "Any proposed sale of the State's stake in Aer Lingus would be of significant interest to passengers and to various interested stakeholder groups, including tourism, business and commerce groups and those working in the aviation industry in Ireland.
"We will have a chance to discuss the implications of any possible sale of the Government's stake in the airline in more detail as well as the impact of any sale for the taxpayer.
"Committee members will want to address concerns about possible job losses and routes being negatively impacted."
Mr O'Mahony said that there are also concerns within the Fine Gael party about the possible sell-off of the Government's stake in Aer Lingus.
Representatives of IAG met Government officials yesterday.
The UK-based airline is also engaged in a due diligence with Aer Lingus, which allows it examine the Irish company's accounts.
Shares in the airline ended the day 5.8% lower in Dublin trade today, finishing on €2.23.
Elsewhere, Tánaiste Joan Burton has said the Government would consider any formal offer to buy Aer Lingus by looking at the strategic questions of what the country needs.
Key Aer Lingus issue is connectivity
Ms Burton said the key issue is connectivity into and out of the country.
She said tourism, which is set to benefit from the effective devaluation of the Euro, making Ireland more attractive to US and UK tourists, is vitally dependent on good year-round air services to these key markets.
Ms Burton told a Labour parliamentary party meeting last night that she remained to be convinced of the merits of the Government selling its 25.1% stake.
Minister for Transport Paschal Donohoe met Labour TDs today about the bid by IAG for Aer Lingus.
Labour deputies conveyed their concerns to him about the sale in relation to the protection of existing jobs and the slots at Heathrow.
The minister said that, as a North Dublin TD, he understood their concerns.
International Airlines Group, the parent group of British Airways, Iberia and Vueling, has made an offer of €2.55 per share. Earlier bids of €2.30 and €2.40 per share were rejected.
The board of Aer Lingus said earlier this week that it is willing to recommend to shareholders that they accept the €1.37 billion offer.
Taoiseach Enda Kenny also said the Government will study the economics of connectivity when making a decision on any formal offer for Aer Lingus.
Another issue is the importance of foreign direct investment flows into the country and their role in promoting Ireland abroad.
Ms Burton said FDI companies need excellent connections in and out of Ireland.
Conor Healy, CEO of Cork Chamber said the sale of an Are Lingus was a critical issue for them as Cork was the economic hub of southern Ireland
Mr Healy said he strongly urged the Government to oppose the sale of Aer Lingus in the absence of any legally binding guarantees about the Cork-Heathrow slots.
Meanwhile, Patrick Edmund of the Shannon Group PLC said Heathrow was the single most important gateway to Europe and the only European hub connected to the west of Ireland.
He said one third of the passengers who travel there are on business.
Mr Edmund said that international companies, who had located in the region, had done so because of the air links at Shannon and were seeking assurances that connectivity will continue.
