The Brennan torpedo weighed 3 tonnes and carried nearly 100 kg of explosives. The guidance system consisted of two reels, each holding 3 km of wire, and each connected to one of the torpedo's two propellers. The missile was launched from a rail runway on shore, where a steam-driven winding machine extracted the wires as the torpedo pulled away.  The controller followed the missile's progress by watching a small mast visible above the surface.

The first weapon that could be guided all the way to its target, it had a range of 3 km, and could travel at 40 km per hour, cruising 3 metres below the surface.  Launch stations were built at strategic points around the British coast,  in Hong Kong and Malta, and at Fort Camden near Crosshaven to defend Cork Harbour, where the launch rails can still be seen.  For 20 years, this was the ultimate in coastal defence technology, though so far as is known, Brennan torpedoes were never fired in anger.  They were superseded in the early 1900s by new guns and other military technologies.