The island is located approximately one mile (1.6 km) south of the Bear Mountain Bridge and is separated from the Hudson's western shore by mudflats and freshwater tidal marshes.

[6] Iona Island is transected by active River Subdivision (CSX Transportation) railroad tracks, but is accessible to the public only by a causeway connecting it to U.S. Route 9W in Bear Mountain State Park, near Doodletown.

The hill on the western side, south of the causeway, was also once treated as separate, and referred to as Courtland Island.

Snake Hole Creek originates in the low marshes towards the center of the island and flows southwesterly, then turning to the southeast and finally east in a long crescent to reach the Hudson.

[7] Artifacts excavated on Iona Island are on display at the Bear Mountain Trailside Museum and Zoo.

Grant's mansion home became a hotel, and the investors gradually added an amusement park with a carousel, Ferris wheel, dance floor, pavilion, and picnic grounds.

On November 4, 1903, an explosion at the site killed six workers, and blew out windows across the river in Peekskill, three miles away.

[8] Following World War II until the early 70s, the famous "mothball fleet" of decommissioned warships were moored near the island at Tomkins Cove.