01/06/2026
Around this time during the Great Siege of 1689 the Jacobite forces built a boom across the River Foyle.
It was constructed around the last week in May or first week in June by the French naval engineer Bernard Desjean, the Baron de Pointis. De Pointis was one of the professional French officers who had accompanied the exiled James II to the Kingdom of Ireland where he acted as Lieutenant General of Artillery for the Jacobite Irish Army. He was among the senior French officers wounded in the early sorties beyond the walls by the Williamite defenders of Londonderry although unlike Generals Maimont and Pusignan he survived and recovered.
Being bested by the Williamites whenever they had met in battle, the Jacobites instead intensified their blockade of Londonderry and thus force the city to fall through want of provisions rather than through an assault on the walls. The Boom was built to this end, for it cut the city off from the sea which was the most likely direction relief would come from.
Rev Walker gives us a detailed description of the boom in his account of the Siege. Walker writes it was just downriver from Culmore at Charles Fort (an old civil wars fortification left over from the previous siege) “a place of some strength upon the narrow part of the river, where the ships were to pass; here they contrived to place a boom of timber, joined by iron chains, and fortified by a cable of 12 inches thick twisted round it; they made this boom first of oak, but that could not float, and was soon broke by the force of the water: Then they made one of fir beams, which answered their purpose better; it was fastened at one end through the arch of a bridge, at the other by a piece of timber forced into the ground, and fortified with a piece of stone work.”
Although it it is not entirely clear as to when the boom was built it was complete enough to be a formidable obstacle when the vanguard of the relief fleet arrive in the Foyle in early June. Colonel Richards records it in the Diary of the Fleet. Richards was onboard TMS Greyhound when it attempted to break the boom and was nearly sunk in the process.
Richards also heard rumours from locals who had come aboard the Greyhound that boats were also sunk in the channel of the river which would hazard safe navigation. From the frigate’s mast Richards observed the Jacobites using boats laden with stone which he accessed as confirming the rumour. However, Rev Mackenzie notes that from the city the Jacobites were observed using boats in the construction of the boom.
Richards was an engineer and made an assessment on the boom for Major General Kirk, which might have played a part in Kirk’s apparent hesitation in attempting to break it. However, when relief was brought to Derry the boom proved not to be as strong as it was feared.
After the Siege of Londonderry de Pointis returned to the French Navy and would be appointed Chef d'escadre (the Old Regime French equivalent to Rear Admiral) in 1697, and in 1705 he attacked Gibraltar but was defeated by Admiral John Leake. This was the second time Leake had foiled de Pointis for it was then Captain Leake who commanded TMS Dartmouth which during the Relief of Derry ensured the relief vessels were able to break de Pointis’ boom.
The boom was a simple but effective weapon that definitely prolonged the siege, contributing greatly to the suffering of the inhabitants of the Maiden City in their darkest hour. For a modern observer the location of the Boom is near Brookhall just downstream from the Foyle Bridge, its location is marked to ships by a navigation light which is appropriately called the ‘Mountjoy Light’ after the ship that broke it.
pictured detail from Captain Francis Nevill's map of the siege depicting the boom.
