From the 1918 Publications of the Ipswich Historical Society:
In the 1820s a Frenchman, Gilshenan by name made examination of many localities along the American coast to erect a salt works. Coming at last to the Ipswich Bar at the end of Plum Island, he exclaimed, “This is the best place I have seen for making salt from sea water.”
Gilshenan succeeded in organizing a company with the backing of Francis J. Oliver and George W. Heard of Ipswich. The deeds were passed in April, 1829 and work was begun at once, giving employment to a considerable force of laborers. The Frenchman appointed Daniel Boynton as captain of the laboring squad and it fell to him to martial his little company in military fashion for the march to and from the Salt Works to the houses where they were quartered.
Twenty eight vats were dug into the peaty sod adjoining a canal eight feet wide and ten feet deep. Sea water was pumped up by six old-fashioned windmills to a height of twenty or thirty feet and then was allowed to fall upon a heap of brush through which it trickled to the vats securing thus a large evaporating surface. Being exposed to the heat of the mid-summer sun, the water gradually dried away and crystals of salt appeared in the shallow vats.
The windmills were supplemented by a great overshot wheel fifteen feet in diameter and six feet wide, suspended in an upright position from a heavy wooden frame. It was made to revolve in a very original way: A large bull was confined within the wheel like a hamster in his revolving cage and his walking turned the wheel. Shallow buckets on the outer rim lifted the water from the canal.
Speedy misfortune overtook the ambitious venture. Heavy rains diluted the water in the open vats. Salt in paying quantities was not produced. In an 1830 article in the Newburyport Herald the correspondent wrote that a loss of thirty or forty thousand dollars had been involved, concluded his article with the remark, “The French gentleman with superintendence of the work is a very intelligent man and bears the misfortune with all that buoyancy of spirit so peculiar to his countrymen.” The whole property was advertised for sale in 1832 by George W. Heard including six windmills, a dam and assorted apparatus. No trace remains today.
The next grand scheme was shipping sand for construction. In the late 1800’s fifty tons of sand were being removed from the island each day and loaded in sand schooners. The Ada K. Damon, loaded with sand, famously wrecked on Crane Beach on its first delivery.
Well into the 1900s Plum Island served as a tourist destination with the Plum Island Hotel at the north end and the Ipswich Bluffs Hotel at the southern end. Hunters flocked to the island, annually killing thousands of migrating waterfowl, which in turn provoked a public campaign to protect the island and the birds.
In 1942 most of Plum Island became part of the Parker River Wildlife Refuge, one of America’s first national seashores. The southern tip of Plum Island where the old Salt Factory and the Ipswich Bluffs Hotel once stood is now Sandy Point State Reservation.
Source: 1918 Publications of the Ipswich Historical Society:
Plum Island - The General Court on October 17, 1649 divided Plum Island among three towns, granted to Ipswich 2/5, Newbury 2/5, and Rowley 1/5. The salt marsh hay, sand, and wildlife were valuable assets to the towns.… Continue reading Plum Island 
Plum Island the Way it Was - Published in 1993, this 100-page book is copied with permission from the estate of the late Nancy Weare. Read by scrolling this page, or click on any image to read as a slideshow.… Continue reading Plum Island the Way it Was 
History of Plum Island - The General Court took action on October 17th 1649: "Upon the petition of Newbury, this Court thinketh meete to give & grant Plum Island to Ipswich two parts, Newbury two parts & Rowley to have one fifth part."… Continue reading History of Plum Island 
The Northern End of Plum Island - Nancy Virginia Weare spent 33 years at her family’s summer camp at Plum Island. After the Parker River Wildlife Refuge was established, she moved to a home on Great Neck in Ipswich overlooking the island. In 1993, after Nancy retired, she wrote “Plum Island: The Way It Was.” … Continue reading The Northern End of Plum Island 
The Early History of Plum Island - Nancy Virginia Weare spent 33 years at her family's summer camp at Plum Island. In 1993 she wrote "Plum Island: The Way It Was," which is reprinted on this site with permission.… Continue reading The Early History of Plum Island 
The Commons - When the Town of Ipswich was established, ownership of a house and land within the town bounds carried with it the right of pasturage beyond the Common Fence. In 1788, the commoners resigned all their land interests to pay the heavy town debt incurred during the Revolution.… Continue reading The Commons 
The Cape Ann Sea Serpent - The earliest recorded sighting of a Sea Serpent in North American waters was at Cape Ann in 1639. In 1817, reports spread throughout New England of a sea serpent sighted in Gloucester Harbor.… Continue reading The Cape Ann Sea Serpent 
Adrift on a Haystack, December 1786 - In a northeasterly storm in December, 1786 Samuel Pulsifer and Samuel Elwell of Rowley were digging clams on Plum Island, got caught in the storm, and took refuge in a stack of salt hay for the night. In the morning they found they had been set afloat.… Continue reading Adrift on a Haystack, December 1786 
300 years on Grape Island - Grape Island was once a small but thriving community, and briefly a popular summer resort. In 1941, 3000 acres of Plum Island including Grape Island were purchased by the U.S. government to establish the Parker River National Wildlife Refuge.… Continue reading 300 years on Grape Island Nancy Weare - Nancy Virginia Weare spent 33 years at her family's summer camp at Plum Island. In 1993, after Nancy retired, she wrote "Plum Island: The Way It Was." … Continue reading Nancy Weare 
Clam Battle! - Life Magazine, July 16, 1945: The government had taken over the lands for a Wildlife Refuge, and the clam battle was on. Ipswich hunters were afraid of losing their private hunting reserves. Ipswich farmers were afraid of losing their land.… Continue reading Clam Battle! 
The ghost of Harry Maine - Harry Maine — you have heard the tale; He lived there in Ipswich Town; He blasphemed God, so they put him down with an iron shovel, at Ipswich Bar; They chained him there for a thousand years, As the sea rolls up to shovel it back; So when the sea cries, the goodwives say "Harry Maine growls at his work today."… Continue reading The ghost of Harry Maine 
The steamship “Carlotta” - The excursion boat Carlotta was built in 1878 at Rogers Point boat yard, and sailed from Town Wharf to the Neck and Plum Island for 35 years. The small hotels at Little Neck, Ipswich Bluff and Grape Island were favorite destinations for tourists and locals.… Continue reading The steamship “Carlotta” 
Gathering salt marsh hay - Salt marsh hay is still gathered on the North Shore today. The grass was stacked on staddles to raise it above the high tides, and was hauled away on sleds over the frozen marsh in mid-winter.… Continue reading Gathering salt marsh hay 
The shipwrecks at Ipswich Bar - The Ipswich Bar has a long history of tragic shipwrecks. Its swift currents and shallow waters are especially dangerous during storms, and many ships have gone aground. The hull of the Ada K. Damon sits on Steep Hill Beach.… Continue reading The shipwrecks at Ipswich Bar 
Ipswich Bluffs - The hotel at Ipswich Bluff on the southern tip of Plum Island was a favorite destination of locals in the late 19th Century, who took the steamer Carlotta from the Ipswich wharf with Capt. Nat Burnham.… Continue reading Ipswich Bluffs 
Nuclear Ipswich, 1967-1970 - In 1967, Ipswich was proposed as a site for an anti-ballistic missile base, and in 1970 opponents prevented construction of a nuclear power plant on Town Farm Road that eventually was built in Seabrook.… Continue reading Nuclear Ipswich, 1967-1970 
The Plum Island Salt Company - In the 1820's a Frenchman named Gilshenan organized an unsuccessful salt harvesting company on Plum Island with a 10' deep canal and a bull turning an overshot wheel like a hamster. A large sundial survived for a few decades, but no trace remains today.… Continue reading The Plum Island Salt Company 
The last cottage on Plum Island - (This article was written by Beverly Perna before the cottage was torn down, and has been updated.) An iconic Ipswich landmark, the last privately owned cottage on the Ipswich end of Plum Island, was turned over to the Fish and Wildlife Service and was taken down in 2016. Boaters and Great Neck residents were most familiar with… Continue reading The last cottage on Plum Island 
Historic Ipswich 
My understanding (no documented source, likely wrong:-) is the old salt vat holes became all flooded together (some creek?) and turned into the fresh water pool where hordes of birds nested and which attracted all the “market hunters” of a century ago. That area is now the Stage Island Pool, and is easy to miss even though it’s very close to the surrounding road, Sandy Point Reservation, and the beach. I heard that a few years ago it was so dry around here the amount of water in that pool fell so much it was no longer enough to dilute the effect of all the lead shot those market hunters had left on the bottom, and as a result many birds (some rather rare) died of lead poisoning over a period of a few weeks.
(On a closely related topic, do you know of any maps or photos of the dikes-and-ponds created when the Parker River National Wildlife Refuge first took over? Although simply making some repairs to an existing dike guaranteed the permanence of Stage Island Pool, the other two large pools [North Pool and Forward Pool] on the inland side of the island were created by the brand new Hellcat Dike. At that time using a bulldozer to significantly rearrange the landscape of new wildlife refuges was an accepted “best practice”, and was so unremarkable back then that I’ve never found either pictures or maps associated with the creation of that dike. [In more recent times that dike has been pilloried because it’s “not natural”. …nevermind that most of the permanent fresh water on the island would disappear without those pools, and the migrating freshwater birds that now rely on them would have to go somewhere else.])