Reckonable residence: how is it counted without exit stamps?

I'm curious - how can passport stamps help you prove that you have been continuously resident in Ireland for X years, when there's no passport control on the way out?

Theoretically, one could leave the country, re-enter a year later and claim they were only away for a week (assuming there are no entry stamps in their passport for other countries). People who have more than one passport could easily skirt this requirement.

Am I missing something?

Dan