I think you are talking about deponent verbs. The principal parts of irascor "grow angry", for example are:
irascor, irasci, iratus sum
It is not the infinitive (the second principal part), but the first principal part that ends in r. To understand this, you need to know the inflections of the passive voice. The definition of a deponent verb is a verb that is conjugated in the passive voice but that has active meaning. You should be familiar with the forms of irascor (irascitur, irascimini, irascebamur, etc) if you know the passive voice, even if the verb is unfamiliar to you. If not, then I think you may have missed some major explanation in your textbook.
flebile nescio quid queritur lyra, flebile lingua murmurat exanimis, respondent flebile ripae