Ryder, Rydder
Ryder, Rydder—These two names are the anglicised forms of Markey (O'Marcaigh) in Co. Louth and adjoining Counties, and of O'Marcacháin in Munster and South Connacht, from marcach, a horseman. See Markham for the latter name O'Marcacháin.
Alphabetical Index of Anglicised Surnames in Ireland
See also Woulfe’s Irish Names and Surnames
… and O'Hart's Irish Pedigrees