Complex sentence: Her mother Margaret Whitly had married the 1st Viscount Clanmalier, then an old man, after her first husband's death, while his second wife was Dorothy Molloy, the daughter of Colonel Charles Molloy of County Offaly, whom he married in 1671.
Split into simpler sentences: Her mother Margaret Whitly had married the 1st Viscount Clanmalier, then an old man, after her first husband's death. His second wife was Dorothy Molloy, the daughter of Colonel Charles Molloy of County Offaly, whom he married in 1671.