Hi Alva,
<puts hands up>
Nope, I am relatively clueless about horses, i.e.; -
"They have 4 legs, lots of them in Weterns, Indians ride them bareback, Mongols can shoot arrows backwards while riding them, you can get a bad fall off one, can get a bad kick from one, they like sugar cubes [who doesn't], don't approach one from behind, thoroughbreds can die of a heart attack from shock, ones with broken legs tend to get shot, their owners are the second greatest cause of bad overtaking by irate drivers on Irish roads after caravans..."
And so on - as I said, relatively clueless.
RE: the stables. I'd imagine simple terrace of individual stables could be built for the going rate of say, a garage.
External concrete rendered block walls painted inside, concrete slab with heavy aggregate finish and/or top reinforced to take point loads from shod hooves. laid to a slight fall to outside or a to gully, potable water supply for horses and cleaning out, flat roof and timber half doors, with restraint loops, internal electric light, that sort of specification
However if you're going to have a number of stables off an internal space, this could bump it up to the level of shell industrial costing, with larger heights and spans, 2M blockwork with cladding above, trussed roof [timber or steel], possibly metal decking and timber stalls with intermediate dividers raised to facilitate the big cleanout. Saw one once in Ballyboughal, but they were aspiring to arena status I think.
Cost really does depend on the brief, but I'm sure someone here could give you a square footage cost, failing which any Q.S. should be able to trot you out a figure. The trouble is, as you'll see from some of the housing threads, its possible to build now for say 60-70% of last years cost and its falling as the lack of work and/or contractors having no alternative economies to migrate to begins to bite.
So the simple route would be to plan what you want, then look at ways of achieving it cost effectively. If you're in Wicklow, Kildare or another "horsey" county, you could go to someone with stables you like the look of, and ask them who built them. There may be local builders who know the tricks of the trade who will give you a good price. You'll still need plans to give them an idea of what you want.
HTH
onq