If you have the latest ISG Consolidated Patch (ICS 16_4_5), you should be able to access PL/SQL APIs via HTTP POST.
Oracle E-Business Suite Integrated SOA Gateway 12.1.3 REST Services Update (Doc ID 1998019.1)
I think irep_parser.pl (is it part of IR Parser?) is needed to load custom plsql into Int Rep? I was asking to confirm
Integration Repository comes along with 12.1.3 E-business suite .
It contains all the seeded and custom loaded E- business suite interfaces.
In order to load custom API's/interfaces into integration repository you need to use irep_parser.pl .
Hope that clarifies your doubt.