Complex sentence: He is the second son of the late Edward Plunkett, 20th Baron of Dunsany in the Peerage of Ireland, and he is the heir presumptive of his older brother, the Right Honourable Randal Plunkett, who is the current Baron of Dunsany.
Split into simpler sentences: He is the second son of the late Edward Plunkett, 20th Baron of Dunsany in the Peerage of Ireland. Oliver is the heir presumptive of his older brother, the Right Honourable Randal Plunkett, who is the current Baron of Dunsany.