If the boiler is going to lock out, then it is a burner issue and not a circulation problem. It is also not gone out on the high limit stat.
If gone to lock out, then problem could be a number of things, i.e. photocell, oil pump, nozzle, solenoid coil, motor, etc. No way of knowing unless I was standing in front of the burner, seeing, listening and checking with my test equipment.
As I said it is time to call out somebody. There may be other issues with your overall heating system and it is best to get a professional to inspect and remedy. You could be causing more damage to the system by trying things that should not be without knowledge and experience. The boiler itself maybe air-locked. Did you open the air vent to release the air when you refilled the system. (This of course will be nothing to do with burner lock out)
Get somebody for your area listed here:
http://www.ofteconline.com/locatecompanyIreland.asp