We moved to Bulgaria in September — are we taxed here on the whole year’s worldwide income?

The advice online contradicts itself because there are two separate tests, and your family triggers the less-known one. Test one is the day count: more than 183 days in Bulgaria within any 12-month period, and you become tax resident for the calendar year in which the 183rd day falls. Arriving in September, your 183rd day lands next spring — under this test alone you would be non-resident this year and resident from next year. Test two is the center of vital interests: if your personal and economic life is anchored in Bulgaria, you can be resident from the moment that anchor moved — regardless of day counts. A family that sold its Irish home, enrolled the kids in Sofia schools and kept no dwelling in Ireland has fairly clearly shifted its center of vital interests in September. On a strict reading, you became Bulgarian tax residents from the move, not from day 183. The practical consequences are less scary than you fear. Bulgarian residency taxes your worldwide income from when residency begins — the salary you earned in Ireland from January to August was earned while Irish resident and taxed there; Bulgaria does not reach back and re-tax it. From September onward your remote salaries fall into the Bulgarian net at the 10 percent flat rate, and the Ireland-Bulgaria tax treaty resolves any overlap in the transition months. Make sure you exit the Irish system properly and note Ireland's split-year treatment for arrival and departure years. File your first Bulgarian annual return by 30 April. A one-off session with a Bulgarian accountant to pin down your exact start date is money well spent — see /guides/bulgaria-tax-guide-for-foreigners for the framework.