Without leading this into a conversation about Irish politics, let me try to explain why Corbyn is not a terrorist sympathiser, or an IRA supporter as such.
There is a simple question to be asked when it comes to viewpoint on the politics of the Troubles; are you in favour of continued British rule over Northern Ireland, or are you in favour of unity? That's essentially what it comes down to. Corbyn, along with many other Labour MP's, believe in unity by consent. Corbyn, like others, could sympathise with the reasons why the IRA felt the need to fight against colonial rule. He started to become actively involved with it when it looked as though there could be a transition from unity by violence to unity by peace and politics. He and others, if you like, offered a hand at this time towards the IRA and were influential in their decision to go down the political route, rather than continue with terrorist attacks.
Corbyn has done more for the peace process than the majority of politicians. He obviously doesn't condone terrorism as means to an end, or he wouldn't be considered a pacifist.
He won't get elected, though.