I have been researching my McAulay family in Islay.
My GreatGrandmother, Jean McAulay, was born on 28/11/1828, in the parish of Kilchoman at the location "Portnahaven" and died on 19/10/1891 in Greenock, Renfrewshire. She had 3 brothers, Duncan (b1821 d 1887), Donald (b 1823 d 1849), Gilbert (b 1824, d 1912) and 1 sister Janet (b 1826 d 1849). All from Portnahaven but the familymoved to Garnkirk Lanarkshire by the time of the 1841 Census. Without earlier records for Kilchoman parish, there may have been other children but who knows??
Jean's parents were Alexander McAulay (b 1782 d 1860) & Jean MacPhaiden (b 1787 d 1871), both dying in Garnkirk. I have been able to identify a sister of Alexander, Jean (b 1789 d 1869) who married a Hugh Campbell.
According to Alexander's death record his parents were Gilbert McAulay & Janet Robertson. In the 1811 Tenant's list there is a Gilbert McAulay at No.4 Portnahven.
I have been scouring the Islay records for McAulays and have listed quite a few of them for the late 1700s, but with no Parish records before the early 1820s and no death records before 1855 it is proving impossible to link any of the names to establish families. I cannot say that I have your Duncan in my sights as he left Islay quite early on and McAulay is a relatively common name in Scotland, particularly in Ross & Cromarty and around Glasgow.
Not much help so far, but we are in the same location on a similar timeline, so there might be a connection.