If the UK and the USA end up aligned against each other, Cold War style, after Ireland is independent, then Ireland is going to be America's Airfield One. National chauvinism would lead Ireland to take more and more responsibility for their own defence. This could happen if the Second World War shook out differently, or if Britain goes full 1984 on us.
In practice, this will be a lot of high-tech jobs, as the Irish Defence Forces man radar stations, naval patrols, air forces, mixed with counter-terrorism, as Britain funds and arms loyalists in Ulster. One or two brigades of armour and one or two air mobile brigades to give us a spearhead, with a draft to create a pool of reservists in case something serious happens. Native Irish bomb by the eighties, probably kept in the same kind of undeclared state as Israel's arsenal, for much the same reasons. Ireland's Navy and Air Force would be her main defence, with the Army just around to slow down any beachheads until they can be cut off by air and sea, or until the US Marines get into it.
Not that Ireland could survive long if the balloon goes up, just that we'd make ourselves prickly enough that it wouldn't be all one way. And the North would be a lot bloodier, with at least two great powers directly and deliberately fuelling the conflict, instead of every great power trying to ignore or defuse it.