Complex sentence: Blanchardstown has grown up in the shadow of its near neighbour, Castleknock, and they have a shared common history until well into the 19th century, when their development diverged.
Split into simpler sentences: Blanchardstown has grown up in the shadow of its neighbour, Castleknock. The two have a shared common history until well into the 19th century, when their development diverged.