Edit To what extent, if at all
Given the overlap between the Troubles and the Cold War, were there any concerted attempts by the USSR, Cuba, or maybe even Argentina to interfere in the conflict? It seems odd to me that the Troubles are seldom viewed through the lens of the Cold War as so many conflicts of the time are. Aside from the assistance rendered by Irish-Americans to the IRA, I can't really think of any "foreign" interference in a conflict that enemies of the UK/US/NATO could have benefited from exacerbating.
If one views the conflict between Northern Ireland Unionists (IRA/Provisional IRA/Republicans) as a part of a larger international conflict that is similar to other anti-imperialist conflicts at the time, it would make sense for the Soviet Union to attempt to exacerbate the conflict in Great Britain's backyard. While the majority of the monetary funding came from Irish-Americans and Irish (NORAID, Irish Northern Aid Committee) some aid did come from Soviet agents as they viewed it as a way to both halt the advancement of NATO and of the European Economic Community, both threats to the USSR's sphere of influence.
The Official IRA (a more leftist off-shoot) also wanted Britain out of Irish politics and land viewed the solution as one of economic and social reforms aligned with Marxist ideology. That's the super tricky (hard) part of civil war and sectarian violence: offshoots from groups create even more complicated and violent problems for the larger government to solve. Now Britain was not only dealing with civil war but also with worries of potential USSR involvement. Jim Prior, who served as Secretary of State of Northern Ireland stated fears of the creation of an "Irish-Cuba...a Marixst state that would affect the wider security of Britain and Western Europe."^(1) Soviet influence can also be seen politically (but how much was actual KGB agents vs. political activists simply identifying with Marxists politics of the time is rather unclear) with movements within Northern Ireland and Ireland to join the Non-Aligned Movement. US diplomats and CIA officers wrote at the time that Russian interference may have been there, but just how much was unclear. One diplomatic exchange between a Brit and an American stated that at the least, "the Russians here bear watching."^(2)
Another interesting book that goes over the direct contribution from the KGB to the IRA is called The Sword And The Shield, The Mitrokhin Archive And The Secret History Of The KGB written by Professor Christopher Andrew that covered the breach of information a former KGB archivist (Vasili Mitrokhin). In that book, Andrew discusses an operation by the KGB called "SPLASH" in which a limited amount of arms were provided by the Soviets. Perhaps the more commonly known international contribution, though, is from Muammar Gaddafi in Lybia who was looking to support the IRA's cause against an imperial Britain. Gaddafi funneled weapons and explosives to the IRA. Foreign interference does make sense from the Cold War perspective, as the USSR, the US, and the West were all in a global competition of influence, after all.
2)An Alien Ideology, 220.
Cox, Michael. "Northern Ireland: The War That Came in from the Cold." Irish Studies in International Affairs 9 (1998)