The charge of terrorist sympathizer has been levelled at Corbyn to cover up the government’s failings to prevent and curtail international terrorism.
When Jeremy Corbyn opposed proposed airstrikes on Syria over concerns it could be used as a recruitment tool for international terrorist organisations, David Cameron called him a terrorist sympathizer. Again, when the Manchester Bombings took place, to avoid criticism levelled against his own party, Boris Johnson was quick to draw attention to Jeremy Corbyn’s history with the IRA.
Johnson has, on several occasions, shared photographs of Corbyn with various Sinn Fein politicians from the 1980s and 1990s to illustrate his support for the Irish Republican cause. But these photographs illustrate nothing. Many Fenians met with international politicians, including Bill Clinton and Tony Blair. Politicians were constantly meeting with Republicans in an attempt to find a pragmatic solution to the troubles in Ireland through open dialogue.
The existence of these photos does not prove that Corbyn supports the IRA or the Irish Republican cause. 
Secondly, the UK Conservative Party is enacting a set of double standards when it accuses Corbyn of sympathizing with violent groups. Conservative foreign policy under David Cameron and Theresa May has maintained strong ties with the Saudi government, who stand accused of deliberately targeting civilians in their civil war against Yemen.
In the 1980s, Conservative Prime Minister Margaret Thatcher supported violent militant groups across the globe, from Cambodia to Angola. Any charge of “terrorist sympathizer” against Jeremy Corbyn reeks of double standards from the Conservative Party.
There is also limited evidence to suggest that the perceived links with the IRA played a major role in Corbyn's electoral defeat. 52 of the 54 seats the Conservatives took from Labour were in vote leave areas. Given Labour's emergence as a Remain party, this would indicate that Brexit played a far greater role in Labour's election defeat.