Sorry, I assumed above that you'd used a spray painter (for emulsion paint) to paint the outside of your house and some of the mist had just been accidentally carried on the breeze to unintended areas, not that it was a graffiti style aerosol.
There are specialist graffiti removal solvents such as the [broken link removed] and ranges, but I'd say they're probably expensive. No harm ringing to check though.
Dublin City Council has a [broken link removed] (for public areas, not private houses) and they have some [broken link removed] that might work for various surfaces. However, for masonry they seem to favour power washing.
They indicate that the fresher it is when you tackle it, the more likely it is to come off.