If a period of deflation is to occur, or ever very low inflation, then is obviously more likely that the ECB will cut rates further.
Negative rates on deposits at the ECB will hurt banks who have excess cash deposits with the ECB by encouraging them to park money elsewhere or lend it. Irish banks have liquidity 'loans' from the ECB rather than sizeable excess deposits. The trickle down effect to retail consumer deposit rates should be minimal, albeit, a negative ECB deposit rate is sure to reduce the already low Euribor and Eonia rates, ever so slightly, which has its own trickle down effects. In a nutshell, I think a negative ECB deposit rate will have a very minor effect on retail deposit rates.
However, the general direction of deposit rates, going downwards, is sure to continue as Irish banks seek to build greater net interest margins. However, the scope for further retail deposit cuts is starting to get limited.
In a deflationary scenario is it correct that the longer I hold on to money the more it will buy?
Yes because your purchasing power increases for the duration of a deflationary period.
Does it make sense to put any funds I don't see myself needing in the next 5 or 10 years into a very long term deposit, even at the current pitiful rates?
If deflation occurs in a prolonged and meaningful manner then long duration term deposits, if they pay well, are normally the most sensible option. However, given that rates are so pitiful, even for 5 or 10 year terms, it is difficult to justify locking your money away for such a long period. Also, if deflation / low inflation does not stick around, and rates in the medium term increase, then it is not the best strategy.