I think you will struggle to find any hard information on your ancestor. You didn’t mention what religious denomination he was but none of the churches in the Skerry area has any records for the 1600s or 1700s. There are few other sources to refer to. Research in Ireland in the 1600s and 1700s is notoriously difficult due to the lack of comprehensive records.
I searched the on-line gravestone inscriptions for that area but did not find any relating to Miller/Millar in the 1600s or 1700s. The name is very common in the area.
I am pleased that you are proud of your Irish heritage. However you probably need to know that Millar isn’t a native Irish name. MacLysaght’s surnames of Ireland describes it as: “An English name very numerous in Antrim and adjacent counties.” In practice the vast majority of the population around Skerry originated in Scotland (only 20 miles away) and arrived in the 1500s & 1600s. So Robert may have been the first generation in Ireland. Indeed he might even have been born in Scotland.
The Ordnance Survey memoirs for Skerry in 1833 said: “The southern parts of this parish are peopled by the numerous descendants of the Scottish settlers, whilst the glens of the mountains are inhabited by the native and original inhabitants of the country, among whom the Irish or Erse language is spoken by them all. The Scotch inhabitants as they are called speak a mixture like Lowland Scotch.”