Irish Nationwide has very strong reserves so it's unlikely to get into liquidity difficulties.
However, there is no advantage to you in having "a fair amount of money" in the Irish Nationwide when you could have a UK Government guarantee with Northern Rock or triple A rating with Rabobank.
It's probably worth keeping the minimum amount required to get the windfall - €20,000.
This also happens to be the maximum amount for the compensation fund. If you have €20,000 on deposit and the Irish Nationwide fails, you will get €18,000 back - you will lose €2,000. If you have €100,000 on deposit, you will get the maximum which is €20,000 - so you will lose €80,000. (It is argued that the government will have to step in to save depositors, but I would not rely on this.)
What we do know about the Irish Nationwide:
Moody's downgraded it by two notches during the week. Most Irish banks have been downgraded.
There are only 4 directors which is very bad corporate governance. (It is a complete mystery why the Financial Regulator tolerates this.)
It is, in effect, run by one person, Michael Fingleton, which is a very serious issue of corporate governance as well.
A large proportion of its lending is to around 40 property developers, all of whom would be facing problems themselves.
I think Irish Nationwide is strong enough to withstand the difficulties, but as I say, there really is no need to take the risk for amounts in excess of €20,000.
Brendan