Someone said :
Deireadh? That doesn't sound right. Maybe it's ok. Did you use Google Translate?
Criochnu sounds better. But I'm no expert.
I never use Google translate, or only to get the gist of a long text as Gaeilge
before working it with a dictionary.
"Criochnu" is the ending of an action, the completion.
"Críoch" is a limit, a boundary, which when reach leads to ending, as the ending of a film.
"Deireadh" is the end/conclusion. Like the end of the day.
Deireadh and Críoch can be used to define one another.
You would probably be right to use críoch/críochnú in some cases where deireadh tends to be used.
I tend to rely on http://www.teanglann.ie/ga/
and its various tabs (including Irish-Irish dictionary) for traditional terms, http://www.focloir.ie/ga/
for more modern English terms to Irish, as my first ports of call.
for more modern, specialized vocab.
I based my sentence on "thiar i ndeireadh an leabhair, back towards the end of the book." By opposition with "i dtús".
I could have considered "i gceann", but it tends to be the end side, like the end of a stick... or from end to end where it could be the "starting end" or the "ending end", the top or the bottom end, etc
I would never advise using Google translate towards an unfamiliar language: it is only good when translating towards the language you master the most, as you can get the gist of the attempted "translation", and filter out the obvious and frequent mistakes.