An eclipsis or an úrú happens when the sound of a consonant is replaced — or “eclipsed” — by a different sound. This is indicated by writing the new letter or letters in front of the first consonant of the word. These “eclipsing” letters are always written lowercase.
b – mb, c – gc, d – nd, f – bhf
g -ng, p – bp, t – dt
This is a lesson walk through of the Bitesize Cúrsaí lesson Initial Mutations: Eclipsis which is available to Bitesize Irish members in our course Crash Course: Nouns 1 – Prepositions And Prepositional Pronouns.