'May have been up to?' I don't think there was any question of what they were planning, they were known IRA members, travelling on false papers, one had the keys to the car that the Spanish police found a few days later, containing Semtex, timers, detonators, and rounds of ammunition. The cock-up was not arresting them at the border or earlier by the police, which led to the police handing over the operation to the SAS, making lethal force just about inevitable.
And, there wasn't really "mainland" influences involved in that case, it was the Gibraltar authorities that requested help from the UK, as they didn't feel the Gibraltar police had enough experience to deal with the threat, which clearly they didn't, considering they failed to spot them crossing the border. As the SAS was engaged with the IRA in the NI, it was fairly logical that the UK would send them as help, despite Gibraltar not specifically asking for them.
I certainly would like to see a united Ireland, once the majority in the north are in agreement. And, I apply the same principle to Gibraltar, as I am a firm believer in 'self-determination', you can't turn the clock back to before it became a UK territory & erase hundreds of years of history, change should only happen when a majority are in agreement.