Irish doesn't have "infinitives" - you do not use aontaigh to say "to unite the city", for example.
The basic rules for conjugating a verb in the Present tense are laid out in the Tips and Notes for the Verbs: Present 1 skill.
You will also find the full conjugation on any verb in the Grammar Database at teanglann.ie
Apologies, EDIT: "In the dictionary, one looks up a verb then not with the verbal noun, but mostly with the verb root (fréamh briathartha). This is the 2nd person singular of the imperative [...] This is a regular, suffixsless base form. A different case is the verb stem (stoc briathartha) the form to which one adds the conjugations and suffixes. " (nualéargais.ie).
Ok, I'm even more confused. Is the stem = 2nd person singular of the imperative, or is that "a different case" and if so, why does the dictionary show the former?