The search came following a number of incidents in the area in recent weeks.
A shotgun has been seized by Gardaí in Westmeath.
The seizure comes following a number of incidents in Mullingar in recent days and weeks, which have led to local uniform and plainclothes Gardaí carrying out pro-active patrols in the area.
The concealed firearm and ammunition were discovered by officers, with the support of the Garda Dog Unit, during a search of an open wasteground.
They will now be sent to the Garda Ballistics Unit for further analysis.
No arrests have been made as of yet, however, Gardaí are continuing to investigate the matter, and are asking anyone with information to contact Mullingar Garda Station on 044 938 4000, the Garda Confidential Line on 1800 666 111, or any Garda Station.