Looking for Ferdinand and Brigid

I have been trying for some time without success to find where in Donegal my ancestors came from. Roots Ireland came up blank because I do not know the names of any parish or townland they lived. DonegalAncestry were unable to take on a paid-for search.

The only location I came across in my family search on Ancestry.co.uk and Scotland's People was that a Mary McDade (maiden surname Green) died in Milford in 1864 at the age of 65. She was the mother of Bridget Brigid?) McDade, born about 1818, who went with her husband Charles Shields to live in Glasgow Gorbals in 1840 along with daughters Susan (born 1836) and Mary (1839).

Charles Shields was born in Donegal about 1804 to Ferdinand Shields and Susan McMonagle but I have no location for this family. Ferdinand and Susan had another son Ferdinand (born 1810) who also went to live in Glasgow.

I had a fantastic visit to Donegal recently and, although making no progress in locating where my folk came from, sensed a great bonding between the county and Scotland, Glasgow in particular.

In Milford, I spoke to the parish priest who had his secretary check but there were no records applicable. If my great-great-granny Brigid McDade was from Milford I can only imagine the Shields family she married into might be from somewhere nearby.

It was all a long time ago but I would appreciate any hints at all. Which will give me a reason to make a quick return to Donegal.