Well, the announcement has apparently tightened the CDS spreads on the banks a lot, so it's now not worth 10bn(?).

According to the Irish Times:

THE GOVERNMENT'S emergency legislation to guarantee the Irish banking system will allow the State to take a stake in any financial institution that receives financial support from the exchequer.

The Bill gives the Minister for Finance wide-ranging powers to protect financial institutions and allows for competition law to be set aside to allow bank mergers, if deemed necessary to protect the stability of the financial system.