Most of the McManus families that I know of come from County Cavan, Ireland.
About 1850, an Owen "Red Oiney" McGovern married an Anne McManus in Templeport Parish, Co. Cavan, Ireland. They lived in Derryvella. Many of their descendants went to Philadelphia.
One of their sons, Peter "Red Oiney" McGovern married a Sarah Sheridan of Creeny, Milton, Belturbet, Drumlane Parish, Co. Cavan, Ireland. Her parents were Bernard Sheridan and Mary McManus (We are not certain if Anne and Mary McManus were related). Peter and Sarah McGovern married in 1883 in Philadelphia, PA.
In the Distribution of surnames in Ireland in 1890, I found:
mcmanus Total births:138
Principally located in counties:Fermanagh.
Spelling variations McManus(129)
Total Ulster = 89 Total Leinster = 19 Total Connaught = 28 Total Munster = 2