Dublin is challenging the $14.5 billion judgment as a "seismic" attack on the integrity of its corporate tax system.
The European Commission says Ireland improperly allowed Apple to avoid paying taxes on almost all profits from sales in the EU.
The agency is investigating whether Facebook understated its tax liability by valuing intellectual property it sold to an Irish subsidiary too cheaply.
The change may ease criticism over Facebook's overseas tax arrangements but won't necessarily mean a tax windfall for the U.K. government.
European tax authorities have been cracking down on U.S. tech firms who they say have not been paying their fair share of taxes in Europe.
The $20 billion combination is likely to revive the "vexed political debate" over U.S. firms using inversions to reduce their taxes.
Investigators reportedly found a $961 million gap between Apple's sales in Italy over a five-year period and what it paid in tax.
CEO TIm Cook affirms Apple's commitment to Ireland, where the company is expanding its Cork campus to accommodate a growing workforce.
Microsoft is challenging a U.S. search warrant for customer emails stored in a data center in Ireland.
The company's Irish subsidiary no longer has to report basic financial data or the amount of tax it will pay.
Popular with companies whose chief asset is intellectual property, the evasion tactic will be closed in 2015.
The European Commission is also investigating tax breaks allegedly granted to Starbucks in the Netherlands.
