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Its origins in the United States mainly come from the Reagh family of East County Donegal, Ireland. They were Low Land Ulster Scots who settled in Ireland in the 17th Century around Ramelton.The family would remain in Donegal until the last of them, Samuel Reagh, emigrated to Connecticut in 1923.
The family had split into two lines prior to 18th Century emigration.
The one branch taking a farm at Magherdrummond. This family remained in Ireland & connections remain there. A section of this family emigrated to Nova Scotia, 1765. They are still there today.
The second line of Reagh's moved to St Johnston a short distance from Rameltion in the Laggan valley.
This looks to be the branch the Rhea's of Tennessee descend from, starting with the Presbyterian minister of Fahan Presbyterian Church, Joesph Reagh/Rhea.
Both lines descend from Mathew Reagh, followed by his son Mathew who married Janet Baxter in Londonderry Cathedral in 1687.
Mathew snr was not Mathew 'the rebel' Reagh as many stories incorrectly record. This man did not exist. The Reagh's are not connected to the Campbell family of Argyle Scotland history. Clan Campbell have confirmed this.
Mathew jnr was not Mathew Campbell Reagh, he was just Mathew Reagh. They were simple Ukster Scots farmers & would remain so in the United States.
In Ireland & Nova Scotia the family retained the spelling Reagh, but there are instances of names being altered over time to Rea, Ray, Wray.
DNA testing will be best to sort out any queries.
There was a second Reagh family from Ireland. The MaCarthy Reagh's. This family had also travelled over from Scotland, but many generations previously & were well established in the South of Ireland by the time of the Ulster Plantation.
The Donegal Reagh's would be Scots Presbyterian & the Southern Macarthy-Reagh's Catholic.