[revised april 2015] [others]
there's a million different ways to approach this, all sharing the common 'mechanic' of moving from place to place in 1904 Dublin.
many of these approaches can be created and hosted free at existing dedicated websites, if anyone is willing to invest the time.
at minimum you'll want 18 'places', one per episode. (but if you want to retrace characters' paths, you'll have to include hundreds of separate places.) you'll need a text description of each place that's vague enough to allow anyone to visit it at any point in the game.
(writing these vague descriptions is a good exercise, and making them freely available will allow them to be refined by others, or ported to other game formats. as we add images, this database could gradually evolve towards a VR 1904-dublin wiki.)
Second Life could be very cool but would charge $100s/month rent as I understand it.
There are free Minecraft clones but it seems like a huge amount of work for a very blocky sim.
Do any SimCity-types allow real city street grids? Can you explore them in 'streetview' mode?
ideally we need a platform optimized to make worldbuilding simple, where you can draw streets and paste buildings with a few clicks. it should allow multiple builders with a wiki-like undo.
most of the buildings can be empty/featureless, but a few need detailed rooms. the 1909 map supplies streetwidths and detailed layout.
choose-your-path text adventures [eg] can force readers to follow the book as closely as the author likes, while adding some forward momentum for lazy readers.
fixed-map text adventures [eg] allow more exploration via NSEW-mapped exits, but consequently permit much less fixed plot and require much trickier implementation.
d&d-style or 2ndLife-style platforms might even allow online socializing as you explore...?
an oldfashioned monopoly-style boardgame with multiple players competing will require a pretty complete reinvention of the book, but again it's a usefully educational challenge to try to boil things down to an essence:
SD: latinquarter hat? ashplant
LB: bowler hat, potato
BM: coin? key?
Bella: bustier, highheel, whip, dildo?
Citizen: biscuit tin/dog/eyeball
places: Tower, school, Eccles, Strand, Nighttown, pubs...
maybe each place comes with a table of possible adventures?
vehicles? postal letters? dogs and cats?
SD & LB build social credit in different ways
alcohol or not, food, orgasms
trading small favors (tobacco)
each pair has changing friendship level?
SD LB MB BB BM
LB +9 -9
MB +4 +3
meeting friends cheers you somewhat, making new friends, meeting strangers
where do we put Church/King/art/freedom/Paris/Martha/Gerty?
each character could be haunted by 'ghosts' they have to face?
SD: mother, church, art, sex, 'nets', BM, Cranly
LB: MB-BB, Rudy, outsiderism
a hexmap can clarify geographic relationships:
(do SD and LB start at opposite ends and wend symmetrically toward each other?)