I assume you're referring to the 1952 agreement that the RAF is allowed to intercept unidentified or hostile aircraft in Irish airspace?
That's because the UK does not want Ireland to have an army. Ireland has a long history of standing with Native Americans, Palestinians, and other groups facing colonization. They even have a military base in Lebanon and a very long standing partnership with Hezbollah (Hezbollah was born out of the struggle to take back the bottom third of their country that was occupied by the US and Israel so they are often seen as an anti-colonial movement).
Ireland having any sort of military capacities would directly contradict UK military interests.
Contradicted by the fact that the Irish military forces were entirely equipped with UK-supplied aircraft and vehicles until the 1960s, at which point Ireland turned towards France instead.
The UK never intervened to prevent Ireland acquiring any weapon system, in contrast it was Irish budget frugality that consistently undermined the military.
At present Ireland is considering the purchase of Gripen interceptors, and the UK seems at worst indifferent and probably actually quite relieved.
That's because the UK does not want Ireland to have an army. Ireland has a long history of standing with Native Americans, Palestinians, and other groups facing colonization. They even have a military base in Lebanon and a very long standing partnership with Hezbollah (Hezbollah was born out of the struggle to take back the bottom third of their country that was occupied by the US and Israel so they are often seen as an anti-colonial movement).
Ireland having any sort of military capacities would directly contradict UK military interests.