I wish I could type as quickly as Godmy!
Until the Anglo-Normans invaded, Gaelic was the language of the whole island of Ireland. Its dialects existed in a gradually changing continuum from Kerry to Antrim (and on into Scotland as far as Caithness and Sutherland). There was also a "learned dialect", the standard language used by the learned classes and for writing. It wasn't quite diglossia, but it was tending thus. The loss of the patron class in the mid-17th century all but eliminated the learned language. The common dialects persisted throughout the country for another couple of centuries, but were spoken by an overwhelmingly illiterate population. By the time the Great Famine was over Gaelic was spoken by less than half the population and wedges of anglicisation separated many of the Gaelic-speaking regions from one another. The dialects diverged more and more from one another and those at the furthest extremes became mutually almost unintelllegible (my father, a Donegal man, hadn't a clue what Cork and Kerry people were saying in their allegedly common language).
Revival became a thing in the late nineteenth century, and by that was meant re-gaelicising anglicised areas. The main players in the revival (long story) decided to go with spreading the common dialects among learners rather than trying to resuscitate the learned dialect. The problem was which dialect should an anglophone Irishman learn and how. Cutting another long story short, the government pushed for a standardised Gaelic to be created by scholars, and that is what they put in the schools and what became the language of adult learning materials up till very recently. So now the "revived language" was an artificial dialect which was the native language of no historical Gaelic language community. Add to that the subtleties of Gaelic phonemes (especially palatalised consonants and lots of vowels) not found in English and you begin to see the difficulties.
Lots of native speakers find this standard language almost totally incomprehensible because its speakers often (wrongly) substitute English sounds for Irish sounds, use English turns of phrase in Irish (e.g. pioc suas - pick up) which are totally unnecessary but because of the paucity of their vocabulary they batter on anyway, and use neologisms invented by the authorities and which were born in dictionaries. Native Irish speakers and their language having been excluded from the professions and centres of learning for centuries, haven't kept pace with new concepts and mostly use English words to supply, but make the words conform to Irish grammar in use. It has been said that the native language is an incomplete language because it doesn't possess all the registers of a modern language.
When these two groups meet they can scarcely understand one another yet they need one another if the language is to survive in direct continuity with its past. Native speakers describe these folk as lofa líofa i.e. fluent in bad Irish. e.g. Gerry Adams!