British Airways parent IAG has begun sending out formal offers to buy shares from Aer Lingus shareholders.
IAG's document promises to preserve Aer Lingus as a separate operating business within the group and maintain a head office in Ireland.
The offer also promises to boost connectivity and keep the Heathrow landing slots for seven years.
The terms of the €1.4bn bid, which were agreed last month, will give Aer Lingus shareholders €2.55 in cash for each share held.
Shareholders have until 5pm on July 16 to accept the bid. The Irish airline also told shareholders that a special meeting will be held on the morning of July 16 to seek approval for the connectivity resolutions agreed by both sides.
Earlier this week, IAG investors were urged to oppose the pay packet of senior executives.
Pensions and Investment Research Consultants (PIRC), which advises pension funds on how they should vote at companies' agms, alerted investors to concerns over IAG's pay plans and the perks given to former directors.