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.
