I first heard about this from the pizza effect Wikipedia page:

> The creation of jack-o'-lanterns from turnips (rutabaga) was a Halloween custom in Ireland and parts of Great Britain. Irish immigrants brought this tradition with them to the United States, and adapted it to use native pumpkins instead of turnips. In time, pumpkins came to be used instead of turnips to create jack-o'-lanterns in Ireland and Great Britain. [0]

[0]: https://en.wikipedia.org/wiki/Pizza_effect#Ireland

EDIT: There’s also more information in the article Wikipedia cites [1]

[1]: https://www.irishtimes.com/life-and-style/food-and-drink/hal...