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I located an 1854 marriage record on Roots Ireland for a Thomas McGrath to a Margaret Cox in the Kilfree and Killaraght RC parish.. I'm wondering if this is your Thomas.
The 1858 Griffiths for Kilfree civil parish does not show a McGrath but there is a John Cox in Cloontycarn townland possibly the father of Margaret. The Griffiths for the bordering parish of Killaraght civil parish shows seven McGrath records. Five of the seven were in Derrinoghran townland. Good possibility that Thomas came from this townland.
Have you considered autosomal DNA testing?
Let me know if you have questions.
Date of Marriage:04-Oct-1854
Parish / District:KILFREE & KILLARAGHT
Witness 1 Patrick McGrath
Witness 2 MariaQuin