Hi
I looked into this when i lived in moneyreagh, the allotments act doesnt apply and the plots per 1000 households also doesnt. However the agenda 21 and the healthy living drives do apply and are the reason that belfast council just opened a new site.
Contact their allotments officer (reg something)and i think that the legal blurb about this is on the belfast city council website. We wrote to ards council with six rate payers asking for plots with no luck at all.
There are a few private plots such as at newtown ards (£200 for less than a half sized plot) you also have to find public liabiilty insurance due to the litigious nature of the community.
There are simularly priced schemes at saintfiled and at gransha in co down not sure about other counties. Its really quite riddiculous that you have to be rich to have a plot, indeed on any sunny sat there is a cue of bmws and posh 4x4 parked outside the ards site (becahse they wouldnt want to get their pretty cars dirty),. Belfast list is 250 long (approx 3 years)
good luck with it all, im glad i live in gb now.
x sunloving