Why Ireland cannot assume Britain will provide its air defence

Would Britain harm its own defence to protect the Republic of Ireland if both countries were attacked?

Britain has an interest in Irish security, but limited forces would be used first for the United Kingdom, so Ireland cannot safely depend on British protection.

Defending the Republic of Ireland serves British interests, and Northern Ireland is part of the United Kingdom. Scarce forces would still be assigned first to the people and territory for which the British government is directly responsible. The example uses Britain's limited ground air-defence batteries, six Type 45 destroyers, and reduced availability of Type 23 frigates. Even forces assigned entirely to British territory are judged insufficient for every British requirement. Ireland therefore cannot safely assume that Britain could provide protection on the scale possible when British forces were much larger.