The magic circle firm won the high-profile mandate after participating in a beauty parade at the end of last year. Irish firms William Fry and Arthur Cox are acting as local counsel to the government and Aer Lingus respectively. Aer Lingus has also instructed Davis Polk & Wardwell.
It is understood that Linklaters, which acted for Aer Lingus on its original IPO plans in 2000, unsuccessfully pitched against Freshfields.
The Irish government, which currently owns an 85.1 per cent stake in Aer Lingus, unveiled plans to float the company on both the London and Dublin Stock Exchanges last week. The government expects the company to make its stock exchange debut before the end of the month.
Aer Lingus is reported to be looking to raise between E400m (£269.92m) and E500m (£337.4m) from the IPO, and is aiming for at least 50 per cent of the airline's shares to be held by Irish investors. The Irish finance minister will retain a 25.1 per cent stake in the business.
McCann Fitzgerald Solicitors and Shearman & Sterling are acting jointly for the government's financial advisers UBS and AIB.
The Shearman team is being led by London-based corporate partner Peter King, while the Freshfields team is led by corporate partner Julian Makin.
Aer Lingus operates a fleet of 35 aircraft, with flights to and from Cork, Dublin and Shannon Airports. The company implemented a restructuring plan and changed its business model significantly at the start of October 2001 after it ran into financial difficulty.