I found this an interesting article.
https://www.irishtimes.com/news/politics/sinn-f%C3%A9in-tells-state-watchdog-uk-rules-apply-to-4m-donation-1.4384246
In the Republic, the maximum donation that can be accepted by a political party is €2,500
The execution of this will could possibly be in question as it was stated he left the bulk of his estate “in trust for the political party in the Republic of Ireland known at this time as Sinn Féin”.
Yet Sinn Fein have used their Northern Ireland location to accept the €4m donation as the UK's donation rules do not have a limit from individuals.
Whether there is more to come on this or not it appears the Irish watchdog can go no further.