Iarnród Éireann are now doing Dublin-Belfast day returns for €20, having previously stopped the €18 day returns they were doing a couple of years ago.
I reckon that the 9.35 from Connolly, arriving 11.35, and home on the 20.10, arriving 22.15, would make for a damned civilised day out from the capital, taking in The Crown, The John Hewitt, The Northern Whig, The Kitchen, and a carry-out from the Vineyard. Perhaps with dinner in Molly's Yard as well.
The €20 fare is online only, I think, and isn't advertised on the website that I can see. But if you run through the booking process it'll give you it.