Soviet foreign policy, particularly as regarded the internal/regional politics of other states, was generally a shining example of realpolitik in action. The USSR was constantly calculating and triangulating how shifts in regional balances of power could weaken its rivals (primarily the US, but also the UK and, at various times, China) and thus strengthen its own geopolitical position.

In this respect, the KGB was one of the primary instruments of Soviet foreign policy. The provision — known as 'active measures' — of covert technical, financial, operational and political assistance to (generally left-wing) insurgencies and dissident political parties abroad was one of the KGB's specialties. Among the guerilla movements with whom the Soviet intelligence services worked — albeit to a limited degree — was the IRA.

It was very seldom unselfish support for other countries' liberation struggles that drove Soviet action; rather, it was a finely-honed ability to cause mischief and a good eye for like-minded political movements which could be co-opted or coerced into becoming Soviet affiliates or client movements. That was the thinking behind the USSR's limited intervention in the Troubles.

You have to consider that Marxist and socialist principles were deeply embedded in the Irish republican movement dating back at least as far as the Easter Rising. The nature of IRA/Sinn Féin socialism is complicated and has evolved over the years, but they have always been left-wing organisations. Richard English's excellent Armed Struggle – A History of the IRA has some superb analysis of the republican movement's complex political DNA.

At the start of the Troubles, the small but active Irish Communist Party acted as a conduit for communication between Marxist elements of the IRA leadership and the Soviet Union. In 1969, just as the political situation in Northern Ireland was spiralling out of control, and the IRA was splintering into the 'Provos' and the 'Official' IRA, Irish communist leader Michael O'Riordan was working to broker a deal (Operation SPLASH) by which the Kremlin would supply arms to the IRA. In August/September 1972, the KGB delivered the first of several shipments of weapons to the post-schism Official IRA. Here's the account of the operation from The Sword and the Shield by Christopher Andrew and Vasili Mitrokhin:

SPLASH was a variant of operation VOSTOK, which had delivered arms to arms to [Palestinian militant leader Wadie] Haddad and the PFLP two years earlier. Once again, the weapons and munitions—2 machine-guns, 70 automatic rifles, 10 Walther pistols, 41,600 cartridges, all of non-Soviet origins to disguise the involvement of the KGB—were transported by a Soviet intelligence-gathering vessel, on this occasion the Reduktor. On this occasion, the arms, in waterproof wrapping, were submerged to a depth of about 40 metres on the Stanton sandbank, 90 kilometres from the coast of Northern Ireland, and attached to a marker buoy of the kind used to indicate the presence of fishing nets below the surface. KGB laboratories carefully examined the arms shipment before it left to ensure that there was no trace of Soviet involvement. The Walther pistols were lubricated with West German oil, the packaging was purchased abroad by KGB residencies and it was specified that the marker buoy should be Finnish or Japanese. A few hours after the arms had been deposited on the sandbank, they were retrieved by a fishing vessel belonging to the "Irish friends," whose crew were unaware of their contents. [...] Several further Soviet arms shipments to the Official IRA were delivered by similar methods.

In the event, the Official IRA — with whom the KGB sympathised — were largely sidelined, and it was the (broadly democratic-socialist) PIRA who emerged as the preeminent republican force in Northern Ireland. PIRA forged a long and productive relationship with Colonel Qaddafi in Libya, who provided them with thousands of tonnes of arms over the course of the Troubles. The KGB didn't pursue a more involved strategy in Northern Ireland — for many reasons, but primarily because their strategic priorities were elsewhere in the world, and the UK became an increasingly challenging operational theatre for Soviet intelligence as Security Service counterintelligence doctrine evolved.