The first National University of Ireland senator appointed to the Council of Europe, he received international coverage for his role in defeating the McCafferty Report which sought to limit the right to conscientious objection for medical staff in the case of abortions.

[17] The motion focused primarily on facilitating personal choice on dying at home and also on making end-of-life care a core hospital competence.

[26] Speaking to the motion Mullen claimed that "implicit sometimes in our thinking on this issue is the view that policy focus should centre on those with the majority of their lives ahead of them; that dying is a taboo subject better skirted around for the sake of avoiding awkwardness and offence; and that as people gradually lose memory, consciousness, bodily control and even hope, they also lose their dignity".

[27] Mullen involved in a controversy in April 2012, when he was accused by abortion rights activists of being "extremely unsympathetic" at a meeting with women who recently travelled outside Ireland to have their pregnancies terminated after they were diagnosed with abnormalities "incompatible with life".

[31] In December 2012, Mullen expressed his sympathy with the victims of the Sandy Hook shooting but warned that his colleagues should "not slip into a double-think where we forget a whole category of children in our own country", in reference to proposed legislation on abortion and comments made by Frances Fitzgerald, the Minister for Children and Youth Affairs, with regards the A, B and C v Ireland.

[32] His comments drew criticism from colleagues including Seanad leader Maurice Cummins and Susan O'Keefe who described them as "disgraceful.

"[33] In a TV3 debate prior to the referendum on repealing the 8th amendment to the Irish constitution in 2018, Mullen said that "mental health has no evidence base".

[37] During the Committee Stage debate in the Seanad on the Civil Partnership Bill 2009, Mullen and Senator Feargal Quinn tabled 77 amendments.

Mullen spoke at length on amendments dealing with freedom of conscience in what Government Senators claimed was an attempt to obstruct the Bill.

[38][41] However, other senators claimed that Mullen was attempting to stifle the bill through repetition and filibuster, that the debate "had developed into an exercise in absurdity.

"What we have seen today in the last four hours is not genuine debate but an attempt to obstruct and to filibuster the passing of this important legislation, with which the vast majority of the House are agreed.

[41] The Sunday Business Post noted that personalised attacks on opponents of the Bill, including Mullen, were an unusual feature of the Seanad debate.

In the aftermath of the defeat in referendum of the first Lisbon Treaty Mullen was appointed to a sub-committee on European Affairs charged with investigating the political impasse.

The motion was based on legislation in place in Sweden and Norway and received the support of NGOs such as Ruhama and the Immigrant Council of Ireland.

[51] Green Party Senator Déirdre de Búrca abstained from voting with the Government as a show of support for Mullen's motion.

[52] He was once quoted as saying he did not believe the Irish government should get involved in the rehabilitation of a schismatic British bishop, Richard Williamson, who denied the holocaust.

[53] In January 2010, Mullen replaced the late Deputy Tony Gregory as an independent member of the Irish parliamentary delegation to the Council of Europe in Strasbourg, where he joined the European People's Party (Christian Democrat) group,[54] the largest political group represented in the Council of Europe.

In October 2010 Mullen together with the Chairperson of the European People's Party at the Council of Europe, Luca Volontè, led the way in pushing through 29 amendments to the McCafferty Report.

Christine McCafferty, a former British MP and main author of the original resolution, said during deliberations that she sought to force private and religious hospitals and clinics to perform abortions.

The party is registered to contest Dáil, European and local elections and Mullen has stated they are interested in hearing from prospective candidates.