Before 26 Sep 1586 [his father] Christopher Plunkett 8th Baron Killeen (age 22) and [his mother] Genet Dillon Baroness Killeen were married. They were third cousins.

Before 1608 Jenico Preston 5th Viscount Gormanston (age 24) and [his future wife] Margaret St Lawrence Countess Fingall were married. She by marriage Viscountess Gormanston.

Before 09 Jun 1611 Luke Plunkett 1st Earl of Fingall and Elizabeth O'Donnell were married.

After 09 Jun 1611 Luke Plunkett 1st Earl of Fingall and Susanna Brabazon were married.

Before 21 Mar 1628 Luke Plunkett 1st Earl of Fingall and Eleanor Bagenal Countess Fingall were married.

Before 1630 [his future step-son] Nicholas Preston 6th Viscount Gormanston (age 22) and [his sister-in-law] Mary Barnewall (age 11) were married. He the son of Jenico Preston 5th Viscount Gormanston (age 45) and Margaret St Lawrence Countess Fingall. They were third cousins.

In Jan 1636 [his son] Christopher Plunkett 2nd Earl of Fingall and [his daughter-in-law] Mabel Barnewall Countess Fingall were married. He the son of Luke Plunkett 1st Earl of Fingall and Susanna Brabazon.

In 1637 [his son-in-law] John Talbot (age 29) and [his daughter] Catherine Plunkett were married. She the daughter of Luke Plunkett 1st Earl of Fingall and Susanna Brabazon.

Before 29 Mar 1637 Luke Plunkett 1st Earl of Fingall and Margaret St Lawrence Countess Fingall were married. She by marriage Countess of Fingall. They were half fourth cousins.