The 1868 church marriage record for James Graham and Sarah Speer showed the fathers as David Graham, a blacksmith and Nathaniel Speer,
a gardener.
Ninth April 1865, at
Rosemount, Derry, David Lewis Speir was born, the son of Nathaniel Speir of Rosemount (Gardener) and
Mary Anne Kearney:
https://civilrecords.irishgenealogy.ie/churchrecords/images/birth_returns/births_1865/03573/2316755.pdfOther possibilities I can see are Glendermott parish Church of Ireland baptisms for a Nathaniel and Mary Anne Spear/Spears/Speers of Altnagelvin, he was recorded as a gardener: Mary Ann Spear b. 3 November 1849; John Spears b. 28 March 1852; Nathaniel Spears b. 3 June 1854; James Speers b. 16 November 1860; William Spears b. 8 March 1863.
There is also a church marriage for a 20 year Robert Speer of Londonderry who marrried Eliza Wallace in St Columb's parish (Church of Ireland) on 2 June 1865, his father recorded as Nathaniel Speer, a gardener.