LUXEMBOURG, Dec 21 Ryanair and Aer Lingus face paying back millions of euros in air travel taxes after the European Court of Justice ruled on Wednesday that they had benefited from unlawful aid from the Irish government. Ryanair, Europe’s largest budget airline, said the ruling would require it to repay 12 million euros to the Irish government, but also said airlines would be able to take legal action to claim compensation from the government over the tax.