Farm was once in Perthshire: https://scotlandsplaces.gov.uk/digital-volumes/ordnance-survey-name-books/perthshire-os-name-books-1859-1862/perthshire-volume-69/31
This has the farm in Stirlingshire: https://media.onthemarket.com/properties/506218/doc_0_2.pdf
The second one is wrong.
Port of Menteith is in the parish of Menteith in the historic county of Perth, also known as Perthshire. See this map
https://maps.nls.uk/view/74400119 which quite clearly shows that neither the Lake of Menteith nor Port of Menteith are in Stirlingshire. Or
https://maps.nls.uk/view/91553691 which shows the boundary between Perthshire and Stirlingshire passing south of the Lake of Menteith.
Or take a look at the valuation rolls at Scotland's People, which clearly list Cassafuir in Perthshire (County of Perth).
In 1975, local government was reorganised around a set of new boundaries that in many cases don't relate at all to the historic county boundaries. The historic counties remain unchanged for ceremonial purposes which means that the boundaries of the new local authorities are not the same as those of the historic county if it happens to have retained the historic name. (For example the present Aberdeenshire council area includes most of the historic county of Aberdeenshire plus Kincardineshire and part of Banffshire, but both Kincardineshire and Banffshire, the rest of which is in Moray, remain unchanged for ceremonial purposes.)
Since 1975 there have been no local authorities in Scotland whose name includes the word 'county'.
Menteith came under the new Stirling District, which was grouped with others to form Central Region. In 1995 there was another reorganisation which abolished the regions and made many of the former districts into standalone local authorities. However this reorganisation did away with the term 'district' as a local government unit, so one now has to refer to the land that comes under a particular local authority as "xxx council area".
As you can see from Stirling Council's web site, it does not refer to its area as 'Stirlingshire'. Nor does Perth and Kinross Council refer to itself as 'Perthshire'. Both of those are historic and ceremonial counties whose boundaries remain unchanged since 1975.
Therefore the estate agent who wrote that advertisement made the error of assuming that Stirlingshire and Stirling council area are synonymous. They are not. Port of Menteith is in the Stirling council area but it is not, and never has been, in Stirlingshire.
The change in local government is a perfect pest for historical research including family history, but all you need to remember is to stick to the historic counties and parishes, because that is how information was recorded until the change just 45 years ago.
All very confusing, even to most Scots!
