New court filings from a coalition of aviation unions in the US have appealed against the US Department of Transportation's decision to allow Norweigan Air to operate flights between Ireland and the US.
They say the decision "systematically violated virtually every basic rule of treaty interpretation."
Unions have argued that Norweigan is trying to avoid the US's tougher labour rules by setting up an Irish subsidiary.
"Opponents claim that Ireland is simply a 'flag of convenience'. In reality, NAI is headquartered in Dublin with 80 employees, 37 aircraft registered in Ireland, and already operates flights to and from Ireland, with many more routes planned," the company previously commented.
From July, Norwegian will launch a series of new low-cost transatlantic routes from Cork, Shannon and Dublin.
The flights will serve smaller airports on the US east coast offering access to the New York, Boston and New England areas.