I am told there are dialects where the distinction between “de” and “do” has been lost and that both have been replaced by “dhon/ghun.” Distinguishing between “de” and “do” is one of the pleasures of An Caighdeán Oifigiúil, the Official Standard.

I’d say the prepositions “do” and “de” have merged in all dialects except maybe Muskerry (West Cork): they use “do” for both “do” and “de”. (And that isn’t new because even in the Old Irish texts (more than 1000 years ago), there might be a confusion between them sometimes…)
However, most dialects still keep the distinction in the prepositional pronouns only (dom/dìom, duit/dìot, etc).