Aer Lingus has released a statement, saying Ryanair's bid to takeover the company undervalues it.
It has advised shareholders to take no action in relation to the offer.
It said management had "transformed" the company into "a robust, profitable airline".
It added: "Aer Lingus has a proven business model and a strong balance sheet including cash of in excess of €1 billion at 31 March 2012, leaving it well positioned for the future."
Ryanair is offering €1.30 per share. Aer Lingus shares are up almost 15% to €1.08 today.
Previous bids foundered on competition issues, but Ryanair insists that circumstances have changed and the takeover should be approved this time.
The European Commission has said it has as yet received no notification from Ryanair or the Government on any takeover bid by Ryanair of Aer Lingus.
A spokesman said the previous decision by the European Commission competition authorities to block a takeover bid in 2006 was "well known".
Any new notification would be examined, a spokesman said.