Don't wish to be a kill-joy, but how are they going to make money with a J31 for which the operating costs must be astronomical?
To make money the fares and going to have to be much higher than could be bought from either EMA or LBA to Belfast, and the journey by road from Sheffield to EMA isn't too bad - except in the height of the rush hour.
To be fair however, the outbound timings from Doncaster to seem to be reasonably good from the business traveler, less so in the other direction, but without a more frequent service that will always be the compromise.
It seems as though LinksAir are looking the be another Eastern Airways - is there room for them in the market? Time alone will tell.