Baron Sydney of Leix is in Baron Ireland.
In 1768 Dudley Cosby 1st Baron Sydney [aged 38] was created 1st Baron Sydney of Leix.
In 1773 Dudley Cosby 1st Baron Sydney [aged 43] and Isabella St Lawrence Baroness Sydney were married. She by marriage Baroness Sydney of Leix. She the daughter of Thomas St Lawrence 1st Earl Howth [aged 42] and Isabella King Countess Howth.
On 22nd January 1774 Dudley Cosby 1st Baron Sydney [aged 44] died. Baron Sydney of Leix extinct.