The idea that no one thinks will work would be that if you collect stamps from each of the brew pubs in Ireland you get a free pint. No one will go to Kerry, Galway, Cork and Dublin to get a free pint. So the cost to any pub involved would be minimal.
The disloyalty card would make the point that your trying to get people to try different things. That diversity is good and something Irish craft breweries value. Though not enough to carry out a scheme that would actually cost them.