I am not familiar with Dennis Martin Ryan (he does not relate to me) but checked Ancestry.com to see if there was any information that might help you. There are 128 trees for his family on Ancestry. Per some of the trees, Dennis died in Hastings, Dakota, Minnesota, United States where his wife, Mary Devaney, and him raised their family. There is a gravesite on Find-A-Grave. Thomas Dennis Ryan (abt. 1805-1897) and Catherine McCormick (another tree has surname Murphy??) (abt. 1800-1870) are identified as Dennis' parents on the tree. Both were from Ireland and died in Hastings, Dakota, Minnesota, United States. The tree includes an obituary for Thomas.
Summary of obituary- He died at 92 at his son Dennis' house. He emigrated to America in1842 and settled in Dakota in 1851. Perhaps that is when Dennis came to America. He had two sisters still living Mrs. Martin Conley and Mrs. Thomas McDermott, Denmark.
The tree identifies Thomas being born in Tipperary and his parents as Dennis Thomas Ryan and Mary O'Brien.
You might already have this information. Good luck with your search.