Complex sentence: The Barony of Moynihan having become dormant on the death of his half brother in 1991, Moynihan spent five years trying to prove his claim to the title which was complex due to the number of marriages of the 3rd baron and questions over their legality.
Split into simpler sentences: The barony of Moynihan was deemed to have become dormant on the death of his half-brother in 1991, following which Moynihan spent the next five years engaged in proving his claim to the title. This was complex due to the number of marriages of the 3rd Baron and questions over their legality.