Very tricky question - all I can say is that Riggan (a name I've never seen before) looks like a variant of Regan which is Irish Ó Riagáin. In 1600 Irish Gaels with names like O'Regan were overwhelmingly Catholic, though some of their chiefs converted to protestantism. At that time adherence to a particular religion was often a matter of expediency, and members of the same family would belong nominally to different faiths. The "Old English", Burkes, Butlers, Fitzgeralds, etc., also moved back an forth to suit conditions. Later immigrants, such as the Scottish planters of Ulster, however, were staunch Protestants.