I think you under estimate the potential passenger numbers on Manchester and Edinburgh. Both EDI and MAN have over 400,000 passengers per annum from Northern Ireland, and taking 15% share* for LDY leaves a potential of over 60,000 pax p.a., the seemingly magic number for Ryanair to run a daily flight.
Bristol and Newcastle on the other hand have only 260K pax p.a. from N.I and so LDY could only expect to attract 40K low cost pax, too little for FYR for a daily flight but perhaps workable for a 3x per week service. Nevertheless there would seemingly be sufficient numbers for a daily service using a smaller aircraft on a different price/passenger numbers basis and so could be attractive to the likes of Aer Arann.
* 15% is the share of NI pax that LDY had on Glasgow, Liverpool and Bristol routes in 2007 and accordingly would seem the airport catchment %.