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I looked at the 1830 Tithe listings for Athleague civil parish and there were no Fitzgeralds listed. Also, I did find a Francis Fitzgerald baptismal record in 1811 in Athleague with father Patrick but there was no related James, Maria or Bridget baptismal records which implies to me that the father of this group was somewhere else and moved to Athleague post 1830.
I looked on the subscription site Roots Ireland for the marriage record in Roscommon parish but none came up but when I went to the register the record was there as you indicated. Interestingly, Roots Ireland shows a baptismal record for an Esther Fitzgerald on January 5th 1846 (three days before the wedding?). However, I did not find the entry on the Athleague register in January 1846 or in December 1846 so I must have missed the record.
I think your conclusion regarding Tromaun is likely correct.
Sorry I could not add much more to your research.