Re: I wonder how much of the opposition matches mine?
"The easiest way to get out of the UK without a passport would be to take the ferry to Northern Ireland and then walk across the border into Eire. When Scotland leaves the UK it'll be even easier ;-)"
Or just fly here. Or take the ferry here.
There is absolutely no requirement for a passport to go between Ireland and the UK on any mode of transport. The second largest airline serving routes between the two requires one, but the first and third downwards don't, just photo ID of any description. Work door passes are enough.
UKBA don't check on the way out, GNIB do really prefer photo ID but will take your boarding pass on the way in. When you're going back, assuming you fly to an airport with sufficient segregation (e.g. Birmingham) you won't show a thing on the way in; if its not segregated the boarding pass will do (Gatwick).
On the ferry there's no checks of any description at all for car drivers, not done foot in an extremely long time to confirm that.