The drainage works of the twentieth century quietly rewrote the landscape here, shrinking the lough enough that what had been open water became a wet, boggy land bridge connecting the island to the Westmeath shore. The 1837 Ordnance Survey map still shows the island as it was, sitting further out into the lough; by the revised 1913 edition, the two formations, Great Island and the smaller Dysart Island, are already being treated as a single place.

The island's role in the 1641 rebellion gave it a different kind of name for a while. Writing in 1682, Sir Henry Piers of Tristernagh recorded that the island had been fortified by the Irish, used as a garrison, and made one of the chief repositories of their wealth. It did not hold indefinitely. Piers describes how the English took it on negotiated terms, only for a man named Ryling of Mullingar to betray the new governor by handing over his cots, small flat-bottomed boats, to Irish captains. Under cover of darkness, those captains landed their men and retook the garrison by surprise. The account has the texture of an old grudge being set down carefully. A later writer, Woods, noting the place in 1907 under the name 'Fort Island', adds that Parliamentary forces seized it on two separate occasions before holding it through to the Restoration. The fort recorded here is described as star-shaped, a design associated with early modern artillery fortification, and it was listed as a protected monument in November 1983.

By 1982, a survey found no surface remains visible, and the site today is largely inaccessible, its interior buried under trees and thickets. Some angular banks are visible close to the shoreline on the lough side, though whether these are the remnants of fortifications or simply the result of water eroding the bank over centuries is genuinely difficult to determine.