In English, we're used to the imperative forms of verbs when we learn them (sit, speak,, or be). The verb is 'sit', not sitting or sat. In Irish, this is also usually true, especially in how an Irish-English dictionary is organized.

However, it's rare to see the imperative form by itself, except as a command or instruction (which can be considered pretty rude unless you're talking to your dog). To find the "base" form of the verb from the present tense version, which is commonly seen, simply remove the present tense ending (usually -ann, -eann, -íonn, or aíonn).

Another simple rule is that the plural of the imperative form is made by adding aigí or -igí.

Bí ciúin! — Be quiet (when speaking to one person)
Bígí ciúin! — Be quiet! (when speaking to more than one person)