Michael Fallon has become the first British defence secretary to make an official visit to Ireland.
The Conservative Party MP was in Dublin Castle to sign a ground-breaking security co-operation deal between both countries.
Under the agreement, Irish and British forces will work together on certain military training, exercises, procurement and reforms.
The Irish Army will also train their British counterparts in peace-keeping operations.
Mr Fallon said the deal reaffirms a resolve to build on and strengthen existing strong links between both countries' armed forces.
"Looking to the future, this agreement will importantly help us both to improve our defence and security cooperation, including conflict prevention and crisis management," he added.
Both armies have already worked together on peacekeeping missions in Mali during 2013, as well as in Sierra Leone, as part of an international effort to halt the spread of the Ebola virus.
Defence Minister Simon Coveney said the deal would provide opportunities for more joint and collaborative work in support of international peace and security.
"It will also enhance the potential for further joint contributions to UN peacekeeping operations," he added.