See below for a map of St. Peter's Civil parish boundary overlayed on a modern map from the townlands.ie website, this uses Openstreetmaps (OSM) as it's basis. You can also view the other parishes on the same website - see this link : https://www.townlands.ie/dublin/ (right click and open link in new tab/window or copy link) scroll down for the civil parishes, in many cases these are the same as the Church of Ireland Parishes.

St. Peter's Civil Parish - OSM/Townlands.ie
It looks like your Mills/Smyth family moved around quite a bit, possibly related to the occupation of George - married at St. Peter's South city/county giving residences at Pembroke Place and Herbert Street which are to the east of the city - but just inside St. Peter's CofI parish. Giving residences at Henrietta Street and Mountjoy square "North side" at the time of children's baptisms…

Thom's 1880 map - St. Peter's CofI Church (red), Herbert Street (blue)
I looked into the residences for George and Catherine at the time of their marriage - 3 Herbert St. for George is off Baggot Street Lower in the east of the south city, in 1840 (Pettigrew & Oulton) the principle occupant is listed as a Joseph Kincaid esq., he is listed as a land agent in later directories. The houses on this street consist of a terrace of large red-brick houses with 4 stories over a basement/kitchen level - probably large enough for servant quarters on the top floor. Catherine's address at 15 Pembroke place is nearby and the householder is listed as Lord Viscount Netterville.
Working back on the Mills side without some further clues could be difficult as George snr. will probably not be included in directories or land records as he likely lived in accommodation provided by his employer. The Smyth side is probably a better option for further searches a) the bride's father is a farmer so likely to have held or leased some land, and b) he has an uncommon name , Nataniel - although it's uncertain at this point that he was alive at the time of marriage. Nathaniel is not likely to have been a farmer in the area of the marriage as it was built up even back then. A check of Griffith's primary Valuation records shows under 20 possible matches for him, but none in or even near Dublin city - most in Co. Cork, some in Co. Antrim followed by Co. Cavan Co. Waterford and Co. Kerry , these date from 1853/56/60.
Two possible further clues on the marriage records are the witnesses - Herbert Smyth and Elizabeth Smyth, likely relations of the bride … possibly siblings ..