Complex sentence: On 6 May 1820, Vaughan succeeded his half-brother in the earldom, but as this was an Irish peerage it did not entitle him to a seat in the House of Lords.
Split into simpler sentences: On 6 May 1820, Vaughan succeeded his half-brother in the earldom. But as this was an Irish peerage it did not entitle him to a seat in the House of Lords.