I looked at Ros davies website
http://freepages.genealogy.rootsweb.ancestry.com/~rosdavies/ only two had Wigtownshire connections
Philip SAVAGE Esq. Inishargy of Glastry ; married Marenret Campbell of Glastry 18 Jul 1808 in Portpatrick, Wigtownshire, Scotland PRONI T1005/1
Thomas SAVAGE Esq. Ballytrustan of Parsonhall; married Ann Caddel of Belfast 4 Nov 1783 in Portpatrick, Wigtownshire, Scotland PRONI T1005
SERVICE, Ros Davies website has several but nothing showing any clues of the name change in 1861
I wondered about the change of name and what could bring it about? Perhaps a family will or a persuasive mother, mother in law.
On the Savage Surname board at
http://genforum.genealogy.com/ one of the contributors Bernard mentions “ Just for information: there is also a branch of the Savages from Ards in Co Down that settled in Scotland. Initially in Lanarkshire (where they were often confused with Usavages - or other variants - from Lithuania). Most of the descendants of this branch now live in Glasgow.”I searched for Service on this Savage board.
On the Service Surname board at
http://genforum.genealogy.com/ I searched for Savage connections, nothing obvious except Service families lived in Scotland including nearby Ayr.
The most likely in my opinion is in a will, on PRONI Wills database the earliest Service will is 1865, the Savages have wills proven in 1850/60s but eg William a wealthy merchant died 1859
Could the grandmother Mrs Margeret Savage have been born Service?
good luck, Jim