Coinne is a masculine noun meaning appointment.
A is a preposition used to connect a noun (as in this phrase) or pronoun with a verbal noun. It lenites, when followed by a lenitable consonant.
Déanamh is a verbal noun from the irregular verb déan / dein, meaning do. The initial consonant is lenited in this phrase because of the preceding a.
Thus, coinne a dhéanamh: to make an appointment.