There are many parallels between the two groups that are swiftly pointed out if you visit either of their strongholds. There are murals in Northern Ireland such as this one professing support for Basque nationalism, along with other separatist/independence movements like the PLO and PKK. I unfortunately have only anecdotal evidence on the Basque side (their mural game isn't as strong as in Belfast and Derry), but generally support for the ETA and the IRA goes hand in hand on both fronts.

As for actual ties, there were arms deals between the two going back at least to the early 70s, involving a range of weapons from handguns to surface-to-air missiles. Leaders of both groups have used each other as safe houses, and as late as 2008 one ETA leader (Iñaki de Juana Chaos) was thought to be living with an IRA-affiliated person in Dublin. Gerry Adams and other IRA and Sinn Fein leaders were also directly and indirectly involved in the cease-fire and disarmament negotiations between ETA and the Spanish government.