I'm afraid you're not entitled to a refund.
You have booked two separate flights without paying any of the charges for a connection between them (airlines charge more for connections, sometimes on their own flights and almost always between other airlines (unless they are in the same alliance!)).
Aer Arann have given you timely notice of the change of their schedule. That it all they have to do.
If you had booked both flights together, say through a travel agent, then you would be entitled to re-routing, exchange or refund on the whole ticket.
Having said that, as pointed out above, I'm also not sure that Aer Arann do connections any more.