He was communications director for the Libertas Institute during its 2008 campaign against the twenty-eighth amendment of the constitution of Ireland. Turnout was 53.1% and the amendment was rejected by a 53.4% majority. McGuirk's involvement with Libertas continued through 2009, when the amendment was revised and approved by a 67.1% majority nationally with 59% turnout. During the campaign, McGuirk tweeted a photo of pro-choice campaigners at a march, who were carrying posters featuring the logo used by the British Union of Fascists in the 1930s. It later emerged that these posters had been handed out to marchers who did not recognise the symbol, by pro-life campaigners as part of what was described by the Sunday Times as "a dirty trick straight from Richard Nixon’s playbook". When Libertas contested the 2009 European elections with three candidates, McGuirk allegedly issued a press release attacking the Simon Wiesanthal Centre without the approval of the relevant candidate, and later attacked the same candidate in strong personal terms in a Facebook post, before apologising.