It was a hornwork, a type of outwork used in siege warfare consisting of a central curtain wall flanked by two projecting half-bastions, designed not to hold ground permanently but to protect a specific point, in this case a pontoon bridge, from a sudden counter-attack. That it existed at all is known largely because the soldiers who built it wrote it down.

The siege of Limerick in 1691 was the final significant military engagement of the Williamite Wars in Ireland, and the crossing of the Shannon was one of its more logistically demanding episodes. General Ginkel, commanding the Williamite forces, needed to bring troops around to the Clare side of the river to press the town from the north-west. On 15 and 16 September, a first temporary pontoon bridge was constructed using the small island of Illaunaroan. A second, more permanent crossing followed downstream, and it was to protect this bridge that the hornwork was ordered. Colonel Michael Richards, a Williamite officer who kept a detailed journal of the campaign, recorded on 19 September that work was pressed hard on both the new bridge and the hornwork on the far bank; the bridge was finished the following day and the earthwork completed shortly after. George Story, whose 'History' of the campaign was published in 1693, noted that when the Williamites withdrew to camp they left a guard in a fort newly cast up on the other side to secure the bridge, and his book includes a map of the siege that shows the hornwork with its two forward bastions clearly marked. A later map by William Eyres, drawn around 1752, shows the locations of both pontoon bridges, though it is thought that much of what Eyres depicted relating to the 1691 siege was drawn from Story's published account rather than independent survey. Beyond these two sources, the fortification does not appear on any other map of Limerick, and no physical remains have been recorded on the ground.