This is not a general purpose guide to the pronunciation of the name Dário.
If you're not a native romance language (Portuguese/Italian/Spanish/French etc) speaker, I would prefer that you pronounce my name as it would (most likely!) be pronounced if it was an English word.
Dar-i-o, with Dar as in dart, i as in free, and o as in throw.
If you are a native speaker of one of those languages, please pronounce my name as it comes naturally to you.
This preference often surprises people. The reason is that trying to get people to pronounce sounds that don't exist in their native tongue (or that are slightly different in their native tongue) tends to lead to people individually coming up with their own slightly different pronunciation of my name. That affects my ability to pick out people saying my name in crowded situations (e.g. in noisy crowds, or across a busy room), because so many different sounds could be a part of my name.
That's also why I don't provide a pronunciation for romance language speakers. They will typically have encountered my name before and have a consistent pronunciation that I've heard before, and having them change it for me would likely introduce new pronunciations I'm not familiar with.