Hello Corsisande et al.
I am not sure if you are aware of Google Books. Many books have previews and some full text.
This one has a preview with snipit view and may be of particular interest to you as your grandfather worked there:
Around and about the Customs House by Jane Meredith (1997)
Includes illustrations. Some subjects are Custom House (Dublin Ireland); Architecture
http://www.rootschat.com/links/07lg/ Seanod this may interest you:
Where's where in Dublin: a directory of historic locations, 1913-1923 : ...
http://www.rootschat.com/links/07lf/ page 6: 55 Amiens Street ...
page 55: ... On 21st May 1921 a meeting was held here to finalize the plans to burn the Custom House. In attendance were Michael Collins, Sean Dowling, Richard Mulcahy, Ginger O'Connell and Oscar Traynor.
Linda