I think at this point you need to use more effort to get the IRC rpg going.
​
If you are tech-savvy, you could setup always online IRC client for the group (for example: https://thelounge.chat/) . There is an IRCv3 initiative going on, which will solve most of the technical issues you are facing, but that will take some time. Maybe in the future IRC will more competitive.
​
I would try to find the people first on IRC, but if there are not enough I would go with discord. If the game is good enough, maybe you can even convince them to switch to IRC later on.