There were at least ten known types of small cannon that were developed, or were thought to have been developed during the civil war. Very little is known of some of these as no pictures or surviving examples have been located. It is impossible to identify from the drawing you posted because inside the bore can not be seen and no measurements can be obtained. However, there appears to be some similiarity of your gun to the 1.5" Ellsworth rifle. The Ellsworth was a breech loader as your gun appears to be. Both have a band over the chase at the trunnions and both have a noticable muzzle swell. Your Tennallytown gun does appear to smaller than the Ellsworth and the trunnion band and trunnions appear to closer to the muzzle. This is only a note of perceived similarities and differances. You need further identifications.
Good luck
Ron