At the age of fifteen MacCullagh entered Trinity College, Dublin. His undergraduate career was one in which he received the highest grade in almost every examination he took.
Sa edad na labing-limang MacCullagh ipinasok Trinity College, Dublin. Undergraduate kanyang karera ay isa sa mga natanggap na kung saan siya ang pinakamataas na grado sa halos bawat pagsusuri siya kinuha.