Good morning, Europe !
In an equally bad English, I will say that IRI is no form from ESSE at all. It is the present infinitive passive of IRE "to go". It is used to build the future infinitive passive, e. g. CAPTUM IRI "to be later caught". When the Latin says CREDO URBEM CAPTUM IRI "I believe that the town will be caught", he thinks "I believe that one is going to catch te town". IRI is an impersonal passive ("one goes") and CAPTUM is a supine denoting purpose after a verb of motion. URBEM here is no subject but a direct object of CAPTUM.
In French : le Latin ne dit pas littéralement "je crois que la ville sera prise" mais "je crois qu'on va prendre la ville".