He did not return to Spain till 1829, and did not again appear in public life, being appointed keeper of the archives at Simancas.
Ó Duinnshléibhe was one of two Donegal men named Father Muiris Ulltach who attended Aodh Ruadh Ó Domhnaill (d. 1602) on his death-bed in Simancas Castle, Spain, with Archbishop of Tuam Fláithrí Ó Maol Chonaire.
Of his original sources, many exist only in manuscript, and his researches in public and private collections of manuscripts at home, and in the archives of Simancas, Venice, Rome, Brussels and Paris, were tireless and productive.
After conducting research work at Simancas in Spain, he successively became an associate and full professor of history at the University of Dorpat (1867).
Simancas |
One of the results was that the Bishop of Zamora, Antonio de Acuña, was executed near Simancas.
His latter years were spent in cataloguing the Spanish manuscripts in the British Museum; he had previously continued Bergenroth's catalogue of the manuscripts relating to the negotiations between England and Spain in the Simancas archives.