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Roots Ireland has an explanation regarding the 1821 census records for Co. Cavan that indicates which 16 civil parishes (out of 36) had records that survived. Clongill was not one of the 16 parishes with records.
The Census of 1821 was taken on 28th May 1821. The original census was destroyed in the Four Courts fire in 1922 but returns of 16 parishes from County Cavan had been copied and stored in Cavan Courthouse until they were later transferred to the Public Records Office – now the National Archives. Returns for the following parishes have survived:
AnnagelliffeBallymachughCastlerahanCastletaraCrosserloughDennDrumlummanDrung and Larah (joint returns)KilbrideKilmoreKinawleyLaveyLurganMullaghMunterconnaught