[1] Interested in learning more about his Irish heritage, O'Murnaghan joined the Gaelic League, through which he became close friends with Douglas Hyde and Arthur Griffith.
[2] Around 1917, O'Murnaghan designed five Christmas cards for Sinn Féin, which are now held in the National Museum of Ireland.
His printed ornamentations appeared in the first issue of the Dublin Magazine in September 1923, illustrating a series by Ella Young entitled "The adventures of Gubbaun Saor and his son".
O'Murnaghan designed the Celtic motif for a new Donegal hand-tufted carpet for the Dáil, Leinster House.
[3] He designed the cover of the 1932 Saorstat Eireann, the Irish Free State official handbook, which was edited by Bulmer Hobson.
[2] In 1922 he was commissioned to illuminate The Book of the Resurrection (Leabhar na hAiséirighe) by the Irish Free State government, with funds raised for this commission through the sale of the Éire page, the winning design which O'Murnaghan had submitted to the selection committee.
He started the work on Holy Saturday, 19 April 1924, completing the first nine and half folios by spring 1927 receiving a weekly fee of 30 shillings.
O'Murnaghan ground and mixed his own paints, in the tradition of medieval artists, basing his designs on earlier illuminated manuscripts.
[2][5][6] O'Murnaghan was interested in archaeology, conducting some excavations at Newgrange, and compared the site with the Egyptian pyramids.