Horner was a supporter of demands for Irish Home Rule and became involved with the rebel factions from the 1916 Easter Rising, joining the Irish Citizen Army.
hˈɔːɹnɚ wʌzɐ səpˈoːɹɾɚɹ ʌv dᵻmˈændz fɔːɹ ˈaɪɹɪʃ hˈoʊm ɹˈuːl ænd bɪkˌeɪm ɪnvˈɑːlvd wɪððə ɹˈɛbəl fˈækʃənz fɹʌmðə nˈaɪntiːnhˈʌndɹɪd sˈɪkstiːn ˈiːstɚ ɹˈaɪzɪŋ, dʒˈɔɪnɪŋ ðɪ ˈaɪɹɪʃ sˈɪɾɪzən ˈɑːɹmi.