It is supposed to be for going to work, tho I don't know how they're ever going to make sure the bike bought is used for that.
To answer the question, bikes bought in the north will probably not qualify because the bike and equipment is bought by your employer tax free and you pay them back for it, either in a lump or by instalments, saving you in turn the tax.
Seeing since the taxes are incurred on sales here, then bikes from outside the jurisdiction won't (IMO) be considered.
Clearly they're buying you the bike for you to use in connection with your work, so in the spirit of the scheme, they are probably prevented from buying you a child's bike or a specialised machine, such as a triathon racer, as you wouldn't be using these to commute.
I suppose it benefits someone who has no bike and wants to cycle, or who has a bike in very bad condition. I think most people already have a bike knocking around in a shed somewhere and it costs very little for repairs - a lot less than the cost of a decent bike - or even to bring a machine that's been off the road for years back into roadworthy condition.
Dave Kane Cycles on the Newtownards Road in East Belfast is a place I know. They do quite a lot of more specialised machines there.
Generally speaking, I wouldn't spend too much because the roads are hard on bikes and they pick up all sorts of muck, scratches and scrapes - you wouldn't want to be precious about it.
Bikes off the shelf in boxes or really cheap, glitzy looking bikes are best avoided. I have a cheap bike (in which I have invested too much in repairs to dump) and find that I've never been able to use all the gears and the crank (bottom) bracket wore out very quickly. It also looks like nothing due to road muck and scratches but is my saviour in traffic, giving me extra time in bed in the morning and cutting through the snarl-ups with ease.