Balfour was Chief Secretary for Ireland from 1887 to 1891. I want to start a discussion of the possible consequences if he had been assassinated by Fenian terrorists sometime during that time.
There would have been widespread condemnation across the political spectrum in Britain and Ireland. Parnell and the Irish Parliamentary Party would have condemned it, whether it happened before or after the Parnellite split of 1890.
There would have been repressive measures in Ireland, but they would have been shortlived because the Conservative government of Lord Salisbury, supported by the Liberal Unionists, would have wanted to return to its policy of coercion and conciliation; of killing Home Rule by kindness. Also a purely repressive policy would have alienated main-stream Irish Nationalist opinion.
The general election of 1892 would have resulted in Gladstone becoming Prime Minister of a minority Liberal government, with support from the Irish Parties ( both Parnellite and anti-Parnellite).
The events of the next ten years are the same as in OTL, except for the absence of Arthur Balfour. I would expect Lord Salisbury to resign as Prime Minister on July 11, 1902 as he did in OTL. Without Arthur Balfour, who would have succeeded Salisbury as PM? Joseph Chamberlain is a favourite candidate on this board, but he was still recovering from the accident in his cab four days previously when his head was badly gashed. Also as a Liberal Unionist, which was the junior partner in the Conservative/Liberal Unionist coalition, he might not have been acceptable to Conservatives.
Four other possible candidates are (in alphabetical order):
Gerald Balfour (assuming that he remained in political life after the murder of his elder brother).
Walter Long.
Charles Ritchie.
William St. John Brodrick.
Whoever became Prime Minister would not have not prevented a Liberal victory in a general election held before October 1907, though it might have been smaller than the Liberal victory in January 1906 in OTL.
Ritchie died in January 1906. Joseph Chamberlain suffered a stroke on July 13, 1906, which removed him from active political life. So if either of these men were Conservative/Liberal Unionist leader and perhaps also Prime Minister, a successor would need to have been chosen.
If another man had become Unionist leader and Prime Minister in 1902, he could have been leader at least as long as Arthur Balfour was in OTL. Bonar Law could still have become leader as in he did in OTL.
Would something similar to the Balfour Declaration of 1917 still have been written?