Unlike the five other Shannon batteries it was built alongside, Tarbert did not follow the standard semi-circular or D-shaped plan; it resembled an obtuse-angled bastion instead, with two faces turned towards the estuary and seven guns arranged along its parapets, three to each face and one at the salient angle. A defensible guardhouse, loopholed for musketry and with a gun platform on its roof, sat at the rear. None of it survives.

The story of the battery is one of repeated neglect and urgency. A payment of £360 was recorded by the House of Commons in 1782 to complete the construction of a battery at Tarbert, with accommodation for artillery and infantry. The works were then left to deteriorate, following what was already an established pattern in Irish coastal defence, until the outbreak of war with the French Republic in 1793 prompted their reconstruction. Between August 1794 and June 1795, new fortifications were erected, including a work named the Pakenham Redoubt after Thomas Pakenham, then Lieutenant General of the Irish Board of Ordnance, armed with sixteen 24-pounders and six 6-pounders. A further cycle of improvement followed after 1803, when the renewal of war led to the most extensive programme of fortification ever undertaken in Ireland. In 1806, Gother Mann, Inspector-General of Fortifications, costed the raising and improvement of the Pakenham Redoubt, including a tower in the gorge, at £3,000. The Board of Ordnance approved the full scheme in 1810; a contractor named Flattery excavated the trenches but failed to complete the masonry, and a second contractor, Quillan, took over and began the structural work in 1812. By 1814 Arthur Watson had been appointed master gunner. In 1872 the fort still held six 68-pound smooth bore cannons and two 5.5-inch howitzers. The gunners finally abandoned it in 1892.

One detail complicates the picture in a quietly intriguing way. A late eighteenth-century painting of the Shannon shows what appears to be a separate battery with embrasures close to the waterline on the north side of the island, distinct from the elevated bastion-shaped work described above. In 1811, Tarbert was recorded as having thirteen guns in total, more than the seven attributed to the main battery alone, suggesting that a second position, perhaps the original battery of 1783 or a work associated with a builder referred to only as Ferrier, was also in use at the same time. Whatever stood at water level is long gone, replaced, like the redoubt above it, by the infrastructure of a different kind of power entirely.