The vote split delegates of the Association of Garda Sergeants and Inspectors (AGSI) down the middle with 13 votes from the body’s national executive swinging the decision in favour 71-58.
A geographical division emerged with most of the Dublin branches, along with the Garda College, voting against the policy, with much of the rest of the country voting in favour.
The AGSI will direct its members not to cooperate with the garda reserve across four areas: accommodation; transport; communications and the role of sergeants and inspectors.
Speaking to delegates at their annual conference in Killarney, Co Kerry, the AGSI president Paschal Feeney said the national executive would draft its plan of action.
He said a letter would be sent out to all members setting out the policy and that senior management would be put on notice of any action. However, he would not state publicly how the policy would be implemented in practice.
This caused considerable concern among some delegates who said they needed to know how the policy would work before they could vote.
Given the tight margin in favour, many delegates afterwards questioned how far the national executive could push a policy of non-cooperation.
Mr Feeney appealed to Garda Commissioner Noel Conroy “even at this late stage” to walk away from the reserve.
AGSI general secretary Joe Dirwan said the reserve had the potential to be the most divisive thing to hit communities since “the Civil War”.
Mr Conroy said he was “a little bit disappointed”.
But he added: “Nevertheless, there’s hope there, there’s hope in so far as it’s an IR (industrial relations) issue. It’s up to us - me as the commissioner and I’ve no doubt the minister - to keep our doors open and we’ll discuss with the various associations and try and come up with situation where they will see it’s a benefit.”
Justice Minister Michael McDowell said he noted the narrow margin and said he was encouraged by the largely responsible tone of the debate.
“I remain confident that it will be possible to reach a resolution,” he said.