University of Glasgow, Library Special Collections: D. S. Robertson Manuscripts
From the University’s website:
A collection of more than 700 handwritten poems and letters personally penned by two Renfrewshire poets, Robert Tannahill (1774-1810) and William Motherwell (1797-1835). Collected by the Glasgow bookseller and publisher David Robertson (1795-1854) and deposited in the University Library in 1907 by Captain D. S. Robertson (1874-1919).
From a different source, the Library has a manuscript collection relating to a third Renfrewshire poet, Robert Pollok (1798-1827).