I am also researching a Mangan family in Loughglynn. I have seen the family name listed as Manning, Mannion, Manion and Mangan.
My great great grandmother's name is Catherine Mangan, she was born in Roscommon County in 1867, died in Rhode Island, USA in 1940's. Her brothers were Thomas and Francis Mangan. Thomas immigrated to the U.S. but Francis stayed behind in Ireland. I have their father listed as Henry Mangan and their mother listed as Winifred Loftus. I have been unable to go anywhere past Henry and Winifred.
I have a piece of paper from when Catherine immigrated to the U.S. in 1888 from a Parish in Loughglynn. The priest's name is Pastor McDermitt. Haven't had much luck tracing anything else on Mangan side.
Perhaps, your Mangan are cousins of my Catherine, Thomas and Francis?