The stock rose as much as 3pc on heavy trading before falling back to close at €1.22 for a gain of 1.7pc.
The shares have now hit their highest point in a year on successive days.
Competition authorities in the EU have twice blocked Ryanair from taking over Aer Lingus on competition grounds and up to now investors have expected the same outcome for this bid.
Yesterday, however, EU Competition Commissioner Joaquin Almunia said Ryanair would be making renewed efforts to ease potential competition issues "in the coming days".
The budget airline, led by Michael O'Leary, has already offered to sell off most of Aer Lingus's landing slots in London's Heathrow to allay competition fears in Brussels.
A spokesman for Aer Lingus declined to comment on the trading, but market watchers said it was not clear if a single investor was building a position in the airline.
At 2.3 million shares, it was the highest volume for a single session since Ryanair made its bid in August.