"What do we say?"
Translation:Cad a deirimid?
I thought deir was irregular. Do you conjugate it when using other pronouns?
abair is an irregular verb - the stem is deir in the present tense, and just as with other present tense verbs, the first person singular and plural use the synthetic constructions ending in -im and -imid in the present tense.
If you don't have the terminology to understand what role something plays, then you'll just have to learn the language the way you learned your first language, and take it on faith that you use a between cad and a verb.
Atá includes the verb tá, so your sentences has two verbs,is/are and we say. Use just a when you already have a verb.