A lot of the Irish ones are supposed to date from the early Christian period, built in response to Norse raids. I'm not so sure myself. The largest concentration seems to be around the Armagh/Lough/Down border region and down into mid-Louth. These areas are away from major waterways that you'd expect the Norse chappies to travel along. Obviously, they could have come storming across the land, but it's more likely that they were built by one kingdom in particular in response to attacks from others (I think). The more limited use in other areas that were definitely attacked regularly by the Norsemmen.
All the above could be bollocks of course. There's a great book on Irish souterrains that I think is now out of print. Always regret not buying it when it was first published.