I'm not sure I can be of much help as Millar intrigues me too. My great grandfather was Millar Thompson. There is a document naming William and Thomas Millar and Richard K Thompson - married at the time to Hannah Magee - it concerns a land transaction - also names a Mr Cupples.
Some Millar must have been important to Richard Thompson for him to name his son after him.
Millar named one of his sons Miller, and his first son called one of his daughters Millaris.
Millar was born in Belfast but Richard lived in Woodburn just outside of Carrickfergus and I suppose was buried there?
If any of this seems pertinent please don't hesitate to contact me.