JoeRoberts, I think if you keep savings in the same bank you have a mortgage with, and that bank goes under, your savings will probably be offset against your mortgage - i.e. your mortgage will be reduced by the amount of your savings but you will have no savings left that you can access for a rainy day. At least, this is what happened to British savers in collapsed Icelandic banks.
So if you don't really need your savings and wouldn't mind offsetting them against your mortgage, it might be a good idea to keep them in the same bank - that way you won't have to go through the hassle of claiming your money under a guarantee and then waiting for a long time to get it. On the other hand, if your do NOT want to have your savings offset against your mortgage, keep them in a different bank. Might be a good idea to split savings between several banks even if they are below 100K - just so that, if one bank fails, you still have access to your savings in the other banks, while waiting for the guarantee to be honoured. People in Britain had to wait for months before they got they deposits in failed Icelandic banks repaid to them, in the meantime having no access to savings at all unless they also had deposits in other banks.
Somehow that aspect - that you'd need to wait quite a while to get your money under a guarantee - isn't given much thought in Ireland at the moment, at least I haven't noticed anybody mentioning it.