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I searched on the subscription site Roots Ireland and did not locate a baptismal record for Patrick Geraghty with father Luke. I searched for all of Ireland. I also looked at the 1855 Griffiths Valuation head of household listing for Co. Roscommon and did not find a Luke Geraghty record. Possibly he was deceased at that point or he was a very small farmer.
I did find a lead in the 1833 Tithe listings for Tumna civil parish. There was a Luke Gaharty in Brackloon townland in Tumna civil parish. Tumna is just south of Ardcarn civil parish. The RC parish would be Ardcarn/Tumna/Cootehall and records start in 1843 which would explain not finding Patrick's 1835 baptismal record.
I would seek other evidence that could confirm that this Luke is your ancestor.
Have you tested your DNA?