It looks like Brigid Supple aka Biddy Sipple (aka Bríd Suipéil) & James Dunphy (aka Séamus Ó Donnchadha) married slightly earlier than you expected... circa 1810. Garrett / Gerald (actually Gearóid) is a given name handed down from both sides of the family.
Brigid & James were living at Lighthouse ST CANICES from 1811-1815 (there they baptised James Dunphy Jr. on 19-Apr-1811). By 1816, they were residing at Chapel, Kilkenny when their first "Patrick" (died in infancy) was baptised on 09-Apr-1816. Then Walken Street in 1818; Gaol Street in 1820 & 23; back to Chapel in 1831.
James' siblings were:
John Dunphy (m. Jane Britain) of Gaol Street (John Dunphy & the Sipples were close to John & Nancy Morris of Walkin St. ST. JOHN'S)
Patt Dunphy & Garrett Dunphy (also invited to godparent)
Biddy's siblings were:
Garrett Sipple (m. Mary Hoyne in 1815; wit. Michael Brennan) was popular, he had numerous god-children around ST JOHNS
Mary Supple (b. 1708 St JOHNS m. Martin 'Rafter' of Lighthouse ST CANICES; aka (m. Martin Breen / Brian of Pike ST JOHNS in 1826 wit. Anastasia Supple) resided at Windgap ST. JOHNS
Ellen / 'Nelly' Sipple (m. James Fitzgerald) of Black Mill St @ Kenny's Well/ Canices' Well ST CANICES (invited James Carroll & Brigid 'Dunphy' to godparent);
Anne / Anastasia / 'Anty' Supple (b. 1801 ST JOHNS m. Michael Shorthall in 1827 wit. James Fitzgerald & Nancy Ryan; remarried Patrick Carroll c. 1842) also of Blackmill Lane @ Kenny's Well/ Canices' Well ST CANICES (invited Brigid Dunphy to godparent twice; and Canice Fitzgerald; Pat, Ellen & Mary Reade, Betty Neary)
Biddy may have origins in Windgap St JOHNS as that is her sister Ellen's given address [marriage cert 1820]. Mary & Anastasia Sipple's baptismal records both name Patt Supple & Mary Phelan of ST JOHNS as their parents. Only 4 Supple birth records have survived in all Kilkenny. Its a rather unusual 'norman' (de la Chapelle) name in these parts.