Add potatoes to a large pot of salted water. Bring to a boil over high heat and reduce heat and simmer until potatoes are tender, cooking about 15-20 minutes. Drain potatoes and mash in a large bowl. I use a handheld potato masher but you can use an electric beater, just don't over-mash or the potatoes will become gummy. You want to keep some texture.
If using pineapple slices, arrange 6-8 pineapple (depending upon the size of your baking dish) and then cover the top with marshmallows).