Domhnach is the nominative form - Sunday. Domhnach Cásca - "Easter Sunday". *Téann Pól ar Aifreann ar an Domhnach" - "Paul goes to mass on Sunday(s)" ("Sundays" in this context is not actually a plural).
Domhnaigh is the genitive form ("of Sunday"). It is used in the phrase "Dé Domhnaigh", and in phrases like culaith Dhomhnaigh - "Sunday suit" or "oíche Dhomhnaigh* - "Sunday night".
Domhnaí is the plural. I provided some examples of when it would be used in this discussion.