Complex sentence: When members of Fianna Eireann on lookout reported that Auxiliaries were approaching the house, the unit of eleven Volunteers split up into two groups, the first leaving by the front door, the second leaving through the laneway at the back of the house.
Split into simpler sentences: When members of Fianna Eireann on lookout reported that Auxiliaries were approaching the house, the unit of Volunteers split up into two groups. One left by the front door, the other left by the laneway at the back of the house.