It's doable, but it's tight and if you do miss the onward flight the only help you will get will be from Ryanair showing you where to buy a new ticket.

As you don't seem to be a holder of a passport issued by an EU country you have to go to the Ryanair desk which is landside and hence you can't avoid UK immigration. The Ryanair desk not validate your details if you turn up less than 40 minutes before the flight even if you have checked in online & hence you don't fly

If you do hold an EU passport and used it when you booked your ticket you can do an airside transfer and avoid immigration which makes it a LOT easier to make the flight.

I'd be tempted to pay another £20 or so and book the Shannon flight on the 30th as well unless you want to risk paying £100 if you do miss your onward flight