As of today, you can buy your copy of Belfast Battalion at the following locations:
Sinn Féin Book Shop,
Tá sé le fáil i gCultúrlann Mc Adam Ó Fiaich (agus suíomh an Cheathrú Póilí fosta).
Cultúrlann Mc Adam Ó Fiaich
You can order the book direct to your front door online here.
There are also a couple of copies in the library of the Museum in Conway Mill if you just want to browse a copy.
And you can download it for Kindle/ebook here (and preview it below) or order it from Amazon.