The Association of Garda Superintendents (AGS) publicly backed the reserve but said it was a "controversial solution" to the force's resource difficulties.
The support of the AGS is in stark contrast to the position of lower and middle-ranking gardaí, who have adopted a policy of non-cooperation to the reserve.
At their annual conference in Dublin yesterday, AGS president Noel McLoughlin said: "The Government have proposed the establishment of a Garda Reserve Unit - a controversial solution to counteract our current resource difficulties."
But he added: "The idea is eminently sound; it has attractive elements, including the practical expression of democracy working in this field of law and order.
Supt McLoughlin said the primary role of the reserve lay in community policing.
He said there was "no doubt" that the reserve would assist the force, particularly in providing a visible presence to the public.
"This is what the public are looking for, a greater visible presence on the street. In fact, I would go further - what the public want is boots on the ground."
He said voluntary police forces worked well in other jurisdictions, particularly in managing public and sporting events.
The Association of Garda Sergeants and Inspectors (AGSI) and the Garda Representative Association (GRA) are strongly opposed to the reserve.
The Association of Garda Chief Superintendents (AGCS) have said they were not opposed to the concept of the reserve.
In his address, Supt McLoughlin also said that crime was arguably the "greatest threat" to quality of life in the country.
"Why should the public have to endure increasing levels of burglary, assaults, car crime and vandalism?
"Most of the crimes that concern the public are not committed by organised gangs; they are rather the crimes of desocialised individuals who have festered in an atmosphere of zero intolerance," he said.
"It does not require sophisticated policing to counteract them, rather a greater garda presence on the street and judges that strictly enforce the message that recidivism will lead to long periods of imprisonment."
He said there was "no doubt" that there were too many crimes committed by people on bail for other offences.
Speaking at the conference last night, Mr McDowell welcomed the association's support for the reserve, describing it as a "significant step".