Searchable by location, date and subject, many of the images have detailed captions including full names to aid family research.
There are numerous prints of daily life in Northern Ireland down the decades including a St John's Ambulance competition in Belfast in 1935 and donkey cart rides in Bangor in 1932, as well as holidaymakers in Groomsport and fishermen in Ardglass around the same time.
Other Irish images show a family portrait outside a thatched cottage in Gweedore, Co Donegal and May Day in Dublin as well as Trinity College graduates and British guards outside the barricaded Four Courts in Dublin.
Russell James, from Ancestry, said: "Captured forever in time, these black and white prints and photos are a journey through the decades and document almost every aspect of life, from daily routines through to occasions of national celebration.
"This sizeable collection is a historical reminder of an ever-changing world, as seen through the lens of photographers working for the most famous name in the field.
"Scenes of children playing, families visiting a local fair or long-forgotten hobbies are just as invaluable as written records for understanding our ancestors."
The collection can be accessed for free at ancestry.co.uk between March 30 and April 2.