I take it you have emulsioned over it and not actual paint, If so try ordinary white paint first and then two coats of emulsion. or if that doesn't work try this [broken link removed]We had problem with leaking roof... fixed now but there are water stains on a wall in our kitchen. I painted over them but they've come through again.... Anyone recommend a stain block to prevent this happening again.
Have to agree with jab 1 Zinsser is the man for the job, have used it several times and always been a good job.
Is Zinsser available in Ireland? Any of the major DIY stores stock it?