Is the modern Irish language really as authentic as it appears to be or was it partially modified when people readopted the language?

Authenticity is always going to be a loaded topic. It's not the best word, but I get what you mean.

The standard dialect, An Caighdeán Oifigiúil, is somewhat artificial. In attempting to create a dialect-neutral standard for government use, they accidentally created one that heavily favored the Connacht dialect. This is partially because the Connacht dialect is something of a middle-ground between the Munster and Ulster dialects, at least for many features of the language. Many native speakers are very unhappy with the Caighdeán because it's no one's native language. It's not good Irish, at least not to anyone but teachers. But it's still recognizable Irish, and mutually intelligible with the native dialects.

You also have a situation where many of the people teaching the language in schools are not native speakers, often teaching children of native speakers. The result us that the native speakers are forever crying out that their language is dying as it becomes watered-down and full of béarlachas, that is, anglicization.

The reality is that Irish is in the middle of a period of transition. The creation of the Caighdeán is having a massive impact on even the language of children who speak Irish language at home, having learned from their native-speaking parents. The pronunciation is tending towards enunciation, where older speakers spoke much more fluidly. The way Hiberno-English speakers pronounce Irish is also become the accepted way to pronounce even the native dialects.

If you watch TG4 programs, like Ros na Rún, the transition of the language is pretty apparent. Older characters, like the venerable Séamus, speak a very thick, fluid local dialect. His Irish is much like recordings I've heard of speakers from Connemara from 100 years ago. Younger actors (even those from the same area) speak much more clearly, avoiding sounds that don't exist in their dialect of English. They speak like the recordings you'd hear in the CDs included with the Teach Yourself Irish books and make less use of idiomatic expressions. It's all still Irish.