if they nationalised them, they might not have to guarantee the debt on top of the deposits

I agree, ironically; if the government "nationalizes" the 6 "national" (charter, i.e. licensed) banks, meaning purchase equity or bonds, the government will in deed own the banks' debts. The government need not "guarantee"; it is obligated to repay the banks' debts. In general, in legal terms, any "guarantee" otherwise is not executable.

Irish Times:

Tuesday's guarantee offered by the Irish government to its six national banks to safeguard €400 billion of deposits and bank debt

The ratio of deposits (retail plus commercial) to debt is not given. I assume there is a government sponsored agency that reports this aggregated data to the public.

The correspondent implies two distinct insurance mechanisms. One guarantees the banks' demand depositors in the fashion of the FDIC; premiums paid by banks may be calculated to some ratio of non-borrowed level of funds. The second guarantees the banks' debts,  their borrowed funds, in the fashion of the Bailout Bill or commercial, monoline insurance of third-party transactions.

It is the latter case rather than the former, I imagine from which arises claims of competition, especially "first mover" advantage across the pond. As you know, all depositors, foreign and domestic, to US FDIC-insured banks are protected, so to speak.

I'm embarrassed to say that I am surprised that EU banks don't offer depositors insurance. As for debt (bond) insurance, is it possible the government merely means to imitate the US Bailout Bill by offering an extra-layer of protection and incentive to firms to preserve their deposits in Ireland?

Diversity is the key to economic and political evolution.