"What do we say?"
Translation:Cad a deirimid?
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.
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.