I don't like sweet relish but in this recipe it's almost essential.  I have also just diced regular dill pickles (Claussen) and they were good but didn't offer quite the same flavor as sweet relish.   You really gotta try this just the way they show it - - on a wedge of iceberg lettuce.  Normally I wouldn't touch iceberg lettuce with a hot pitchfork because it has no vitamin value at all, but it's good in this recipe and it makes a wonderful start to a corned beef meal.

https://www.simplyrecipes.com/recipes/thousand_island_dressing/