JMM99 in his post above commented (cited in part):
No doubt that the Brits had few indigenous Irish (whether Gaelic-, Norman- or Anglo-Irish) so committed on their side in (say) 1918-1919 as to be penetration agents.

I've thought from time to time about the shift in attitude from (say) 1914 to the 1918-1919 period. In 1914, it was common for Irishmen to flock to the colors, and many Irish before that were professional military...

(Later) So, there is more to it than the 1916 executions of a relatively few revolutionaries; but that was the spark that Collins et al were able to use in igniting the dry wheat field a couple of years later.
I have read a little on Irish history in this period (1900-1921) and it is clear that there was considerable loyalty to the Crown across Ireland at the time of declaring war in 1914 - touched upon in the recently read 'Sleepwalkers; How Europe Went to War in 1914. There had been a royal visit to Ireland a few years before 1914, with extensive public participation. Yes there had been a persistent strain upon the Anglo-Irish relationship, with a small minority favouring violence and the election of nationalist MPs to the Westminster parliament.

In 2013 I read a fascinating article in the journal Intelligence and National Security (Vol. 28 No.4) 'The British State and the Irish Rebellion of 1916: An Intelligence Failure or a Failure of Response' by Geoff Sloan. Amidst all his points there are references to the availability of informers up to the Easter Rising; civilian informers mainly for the police (RIC and Dublin Met Police) and for the Royal Navy (which had several bases).

Yes the Easter Rising was a shock to the public, not the government as Sloan argues. When prisoners were led away they were not cheered.

It appears a lot of Irish opinion changed when a 'few revolutionaries' were executed, although my suspicion is that many became neutral towards the "men of violence" and once the British reverted to military coercion the majority shifted their way. The 1919-21 rebellion was not pleasant; although what followed was even more "no holds barred".

Given Irish history and the crude, coercive reaction to the 'Easter Rising' and what followed years later it is an achievement that the nationalist 'spark' failed to ignite wider public support for a long time.

Sloan points out that effective action was taken to prevent any 'Rising' beyond Dublin and interning the leaders was an option beforehand.

Incidentally the commemoration of Ireland's war dead from both world wars has only recently become acceptable in the Irish Republic. Not to forget the PIRA bombing of a Remembrance Sunday parade @ Einniskillen - a "legitimate" target.