It was originally established in December 1940 by Colonel Michael Joe Costello as a means of disseminating training material among the Local Security Force (LSF), the forerunner to the Army Reserve, but later became the newspaper of Costello's First Division.

[2] In its current form, An Cosantóir is a 40-page colour A4 size publication, issued 10 times per year.

[4][5] An Cosantóir is published by the Public Relations Section, Defence Forces Headquarters (DFHQ) with offices at Ceannt Barracks in the Curragh Camp.

[6][7] It has a monthly readership of around 25,000, and up to 45,000 readers online,[3] and is sold in approximately 200 outlets around Ireland.

[8] An Cosantóir holds an annual 10km road race event in the Phoenix Park in autumn for Permanent and Reserve Defence Forces personnel, Department of Defence staff, veterans and invited guests.