Now that I am back from Easter holidays, I can give my reader Oscar Lacken a more comprehensive answer. What books should be easy for someone who has already finished the Learning Irish course? The obvious answer is:
Bóithrín na hAille Báine, by Mícheál Ó Raighne.
This book is exactly the kind of Irish you have learnt from Mícheál Ó Siadhail’s text book. It is fun, although a little too much fun – I think it would work better in amateur theatre than as a novel. Mícheál Ó Raighne has written two other books, but they are less interesting. The spelling departs from Standard Irish, but in a way you should be familiar with, from the textbook.
Seán Ó Conghaola (also spelt Seán Ó Conghaile) has written at least Saol Scolóige, which isn’t too thick or heavy – 112 small pages. His other book about Gaeltacht life, Cois Fharraige le mo Linnse, is more difficult and demanding, but at least it should be familiar dialect for someone who has learnt Irish from Ó Siadhail’s Learning Irish.
Colm Ó Gaora’s Mise is written in Ros Muc Irish – the best Irish of Connemara. At times, it might be very demanding, but it is also quite interesting, if you are into recent Irish history. The rich language is worth studying.
One writer you should remember is of course Pádraic Ó Conaire, above all his short stories. His Irish is not particularly dialectal or complicated, and it has even been said to be poor – not in the sense “bad”, but in the sense “not rich”. Note though that he migh use old-fashioned verb endings (chaitheas instead of chaith mé and the like).
And remember: you need Niall Ó Dónaill’s big dictionary unabridged. Happily, it is now available on the Web, searchable.