Entry requirements are completely dependent on your nationality. Things can change, so for the most up-to-date visa and passport information, we advise checking the international travel advice for your nationality, below are two examples.
If you are travelling to Bulgaria from the UK on a full British passport: https://www.gov.uk/foreign-travel-advice/bulgaria
If you are travelling to Bulgaria from Ireland on an Irish passport: https://www.dfa.ie/travel/travel-advice/a-z-list-of-countries/bulgaria/