The Dáil has voted by 75 votes to 42 to join a new EU defence network.
The European Union’s Permanent Structured Co-operation Agreement (PESCO) is seen by some as the first step towards an EU army.
It will see countries taking part in sharing information and co-operating on issues of defence.
The Taoiseach Leo Varadkar has said it is necessary to ensure the security of the State, and it will not affect our neutrality.
Opposition TD Richard Boyd Barrett is staunchly against the idea.
He said: “This is what Tusk said about PESCO, its purpose is to protect the bloc from the affects of the migrant crisis and hostile bordering states.
“Affects of the migrant crisis… that’s 35,000 people drowned in the Mediterranean thanks to ‘fortress Europe’, and they want to militarise the wall that Donald Trump dreams of building to keep those desperate people out.”