Complex sentence: The heir apparent of the heir apparent is Lord Mount Charles's son Rory Conyngham, Viscount Slane (b.2010), who is a grandson of the 8th Marquess.
Split into simpler sentences: The heir apparent of the heir apparent is Lord Mount Charles's son Rory Conyngham, Viscount Slane (b.2010). The young Lord Slane is a grandson of the 8th Marquess.