1) This rule is part of the treaty in which Ireland joined the EU - which was the very first thing added to Irish law.

2) That treaty (which is part of Irish law) allows for certain kind of EU laws (called "regulations") to be directly binding without the Irish lawmakers needing to do something actively. Your link covers another kind of EU laws (called "directives") which in fact have to be transposed before becoming directly enforcecable. However that treaty (which, remember, is part of Irish law) forces the Irish lawmakers to actually transpose them. If they fail they open the country up for litigiation which makes directives indirectly binding.