McWilliams has a reputation for explaining economic ideas with memorable phrases or stock characters, most famously "breakfast roll man".

Between 1990 and 1993 he worked as an economist at the Central Bank of Ireland, a period which covered German reunification, the Maastricht Treaty, Bertie Ahern's term as Minister for Finance and Black Wednesday.

Watching these phenomena unfold from the perspective of a Central Bank official shaped his opinions, and these events would be revisited in his writings, especially the triangular relationship between access to credit in Ireland, the retirement funds of an ageing German workforce, and European monetary policy.

[3][4] McWilliams moved to London to work at UBS as a senior European economist and head of Emerging Markets research for Banque Nationale de Paris.

[6] He credited what he described as "an economic miracle" to spending cuts by Charles Haughey's 1987 government, "which laid the foundations for the impressive growth in the 1990s".

[10] In 2000 – the year he had previously predicted that the Celtic Tiger would end -[5] he warned of a "Hong Kong style property market collapse" in the not too distant future, noting that Ireland was experiencing labour shortages, rising asset prices and high borrowing (five times the then European rate and four times the then US rate).

[11] During 2001 Ireland experienced a foot and mouth outbreak which had an impact on the economy, with some commentators declaring the Celtic Tiger to be dead.

In The Pope's Children, McWilliams applied ideas which he saw used with great effect in Bobos in Paradise: The New Upper Class and How They Got There by American conservative writer David Brooks[14] to Ireland, creating neologisms to describe certain economic actors as stock characters – HiCo and Breakfast Roll Man, for example.

In 2010, McWilliams co-founded two annual events: the Dalkey Book Festival with his wife, Sian,[20] and Kilkenomics[21] a festival-style conference which combines economics with standup comedy.

[23] McWilliams conceived The Global Irish Forum, an event modelled on the World Economic Forum in Switzerland[24] and inspired in part by Israel's social and economic ties to Jewish communities in trading partner nations (he has frequently referred to harnessing the power of a 'Global Tribe' ).

[27] While he has refrained from publicly supporting any political party in Ireland, he hinted at running as an Independent in the 2011 Irish General Election and raised awareness of individual candidates such as Dylan Haskins, Evelyn Cawley[28] as well as Stephen Donnelly who would go on to become Health Minister with Fianna Fáil.

[31] McWilliams had stated on multiple occasions in the 1990s that the circumstances were in place to create a housing bubble in Ireland, and had predicted a massive crash in the early 2000s which did not happen at that time; after Ireland recovered from the economic shock of 9/11 and the foot and mouth crisis, and his predictions were dismissed as alarmist, he expanded on his initial theory by describing Ireland's housing boom as a "confidence trick" by "an unholy alliance of bankers, landowners and a pliant political class" which would collapse resulting "in a generation in negative equity"[32] on RTÉ.

[43][44] Despite hoping for a Remain vote and acknowledging that Brexit is a worse option for Britain, financially, he has expressed an understanding for why the British people might have voted for it with regards to issues like the desire for independence,[45] noting that Switzerland and Norway pay handsomely to have access to the common market while remaining independent of the political union because they value the "feeling" of sovereignty.

[47][48] In an article after 2023 Israel–Hamas war, McWilliams stated that "in postcolonial Ireland, we have the most pro-Palestinian population in Europe yet the most pro-American economy in the world.