My IRC client uses about 10 times less system resources then Discord.
I disagree with Edible on the stance that there are no good mobile IRC clients. I use Andchat on my phone and it runs perfectly well. I can run it from the SD card. Lightweight, no ads, has almost all the same features as the reliable Hexchat does on my PC, very little data usage, and it's free. I've never tried to go on PPIRC with it however and I'm pretty sure PPIRC is not friendly with it for some reason, but I don't have trouble connecting to any major IRC networks, or small ones. You might want to bug Suzuran about that if that's an issue.
Also my phone is from like 2010 and it still runs IRC just fine.
To be honest Discord isn't really an IRC alternative, there's nothing that's really an IRC alternative except maybe Telnet. Discord is just too large of a program to really compare to something as lightweight as IRC. While it does enjoy lots of throwbacks from IRC, it's definitely its own thing.
However I will say that Discord was built to be a 'Skype killer' and it definitely wins on that front. Almost everyone I know has migrated from Skype to Discord, and it's a much better, much less bulky and terrible program then Skype. The latency needed is much, much lower and it does all the things Skype does while still being much smaller. It doesn't do video calls yet, however. But only 'yet'.