It's been a while, but I believe there is a chapter in the Terror Network on the ETA and it's Links.
Check also-Gayle Rivers, "The War Againste the Terrorists and How to Win It".
Former Australian SAS. Some of it over the top, but he documents the International links quite well and outlines a strategy that many here advocate-Kill, don't capture.
These books were done in the 80s, but have relevance today. Sterlings also documented the Soviet Financing of many of these groups.
As for the Basque and the Irish movements, they both came out of 19th century Romanticism, which focused on national identities, the revival of dying "national" languages (such as Gaelic, Basque and Catalan), and a rejection of industrialization. While the Irish movement was actually encouraged primarily by Anglo-Irish writers, in País Vasco and Catalonia, many of the proponents of this were clergy, who saw it as a return to the old days of rural life, the tiny, peaceful village, etc., etc.
In the case of ETA, this Basque nationalism attached itself first, strangely enough, to the conservative cause during the Carlist Wars (a fight over succession) in Spain, but later, because Pais Vasco is an industrialized region, became increasingly involved with leftist unions and other similar leftist activities. By the time of the Spanish Civil War, Pais Vasco was famous for being very left wing. After the war, the Basque movement revived in ETA, which was violent from the start, and devoted itself to killing people in Franco's government. The Socialists and all the left went easy on it for that reason, because they saw it as an ally, but after the Transition (after the death of Franco), when the Socialist Party got into power, they found they had created a monster they could not control. In the subsequent decades, they never repressed it as firmly as they should have, with the result that the overtly Marxist organization still exists and has killed more than 1000 people in Spain alone. And it has internationalized, going first to Spanish speaking countries, where its leaders could hide out and raise and launder money.
The goal of ETA in Spain is to separate País Vasco from Spain and establish a separate country, run along Marxist lines by the political wing of ETA.
In the case of Ireland, there actually was an oppressor, and even after the Irish Revolution, Irish Catholics in the North continued to suffer from legal and other restrictions, and they initially saw the IRA as something that would protect them. This, of course, was far from the truth, and the IRA essentially devolved into a combination of criminal enterprise and Marxist cadre.
Both ETA and the IRA collect the "revolutionary tax" (extortion) in their respective areas, work with other terrorist groups who are opposed to the West and capitalist democracy, and dominate through terror, assassinations, torture and kangaroo courts that "try" anyone who objects to their activities.