Slack has an IRC bridge. We use it for Leeds Hack Space. Its transparent. It mirrors real time from Slack to IRC and from IRC to Slack. It would be ideal for this purpose example where I can use IRC when I am at my desk but then use Slack when on mobile.
I would vote we use Slack as it would be transparent.
I would point you to the Leeds Hack Space discussion here regarding Slack:
It sums up the case for Slack well and for Leeds Hack Space I have seen more involvement with members. IRC can still be used and Slack is just essentially a client for IRC.
I have experience getting all this to work if you would like to try Slack. Its free.
We use Slack at work to talk to our internal IRC when on mobile as well.
I prefer IRC but there is not an easy way to use IRC on mobile.