I suspect that the Irish (and EU allies) are possibly also contemplating the worst case scenario of no-deal Brexit, or the UK government refusing to implement the divorce deal obligation to enforce an EU border in the Irish sea. Quite understandable that the Irish (and EU) might not be so keen on UK continuing to look after Irish air-defence in that scenario.
This little announcement might even be a tactical sucker punch to later announce that the EU Defence Agency will form a air defence wing, and rotate various EU AIr Forces through Ireland (just as the Baltic states benefit from rotating NATO cover).