I've been doing some more research where I can of the Mills in Rathdrum and their connection to Sarah Mills who married Thomas Williams of Rockstown in 1739. It's difficult to figure out sons from cousins from brothers from remarriages, but so far my guess would be that there was a Daniel Mills who was the father of the apothecary Daniel Mills (~1693 to 1789), and probably also the father of a Richard Mills (burial record for "Mr Richard Mills" at Rathdrum in 1758, age unknown) and a Joseph Mills (from Cantwell, buried at Rathdrum aged 88, ~1682 to 1770).
A few random bits and pieces:
https://www.familysearch.org/ark:/61903/3:1:3Q9M-CSJW-JJ43No. 31386
1725 - A Daniel Mills of Rathdrum is mentioned, innkeeper. Possibly the father of Daniel, Richard and Joseph, if they were brothers.
https://www.familysearch.org/ark:/61903/3:1:3QS7-89LW-GPDSNo. 302
1732 - Thomas Williams of Rockstown in Wicklow, wife Sarah, eldest son George, middle son Thomas (who would go on to marry Sarah Mills), youngest son William. Daughters Elizabeth Long nee Williams, Febey and Sarah. Grandson George Long, Elizabeth's son.
https://www.familysearch.org/ark:/61903/3:1:3Q9M-CSN1-63YM-MNo. 49225
1732 - Unusually for the time, Thomas Williams (father of the Thomas who married Sarah Mills) bequeaths the entirety of the Rockstown farm to his youngest son, William Williams.
https://www.familysearch.org/ark:/61903/3:1:3QS7-99LW-2ZMLNo. 39
1736 - Richard Mills of County Wicklow, farmer, marries Elizabeth Sherrard of Arklow Parish. Starting from 1738, numerous baptisms of a Richard and Elizabeth Mills are performed at Rathdrum, including their first son which is a Daniel Mills. The last baptism is in 1757, the year before the burial record for Richard.
https://www.familysearch.org/ark:/61903/3:1:3Q9M-CSH3-S3XR-GNo. 65202
1738 - There's a Joseph Mills, son of Daniel Mills of Rathdrum.
https://www.familysearch.org/ark:/61903/3:1:3QSQ-G9LW-KFDHNo. 216
1739 - Thomas Williams, now working as a clothier in Wicklow, married Sarah Mills of Rathdrum. Their earlier children are baptised in Rathdrum, and most later children at the Castlemacadam parish closer to Rockstown. Three of their sons are named Joseph Williams, Richard Williams and Daniel Mills Williams.
https://www.familysearch.org/ark:/61903/3:1:3Q9M-CSJW-GD3TNo. 55584
1743 - This entry seems to be confused and alternates between a Joseph Mills and a Joseph Wills. I think the actual person may be Joseph
Mills, carpenter of Rathdrum, and he is obtaining a property currently inhabited by John
Wills the elder and his son John. There is indeed a John Wills, son of John, baptised in Rathdrum in 1708. Witnessing the article is Daniel Mills of Rathdrum, gentleman.
https://www.familysearch.org/ark:/61903/3:1:3Q9M-CSJM-Q6BQNo. 99141
1744 - Mentions the original lease of Rockstown by the elder Thomas Williams from Rev. Thomas Whaley of County Meath in 1712 and tied to the lives of three people. The lease is now being renegotiated with the late Whaley's nephew Richard Chappell Whaley of Whaley Abbey in Wicklow. In place of William Williams and Sarah Williams (his wife?) who have apparently died somehow, Thomas Williams and his wife Sarah Mills are added to the deed along with the existing name George Williams.
Witnesses to the memorial are Richard Lambert, gentleman, and Daniel Mills, apothecary, both of Rathdrum.
https://www.familysearch.org/ark:/61903/3:1:3Q9M-CSNH-WGT1No. 93415
1749 - Some organising of funds between Thomas Williams of Rockstown, farmer, and George Williams of Dublin, chandler. Presumably the two brothers. Subserving witnesses to the deed are Daniel Mills and Richard Mills, both listed as farmers of Rathdrum.
https://www.familysearch.org/ark:/61903/3:1:3Q9M-CSH3-97F8No. 188713
1768 - Some sublease agreements or somesuch of Rockstown between Thomas Williams and Hugh Byrne and widow Elinor Byrne. A Richard Mills of "Longhouse" in Wicklow is mentioned as a witness.
https://www.familysearch.org/ark:/61903/3:1:3Q9M-CSNH-496P-CNo. 212937
1777 - More lease arrangements regarding Rockstown between Thomas and Sarah Williams and their eldest son Daniel Mills Williams. Also mentioned is Thomas King of Ballynaclash (Ballinaclash), I believe the one who later willed his estate to Daniel Mills MD, son of the apothecary Daniel Mills.
https://www.familysearch.org/ark:/61903/3:1:3Q9M-CSH3-33BH-2No. 379104
1803 - Thomas Williams and his wife Margaret Edge, Thomas being the eldest son of the now late Daniel Mills Williams. Thomas King mentioned again.
https://www.familysearch.org/ark:/61903/3:1:3Q9M-CS54-33S3-GNo. 422309
1809 - Thomas Williams of Rockstown. Mentions Thomas King of Kingston along with a Daniel Williams and Daniel Mills Williams. The latter two probably Thomas' brother and son, respectively.