Cronin initially managed Instinctif Brussels after her business Policy Action was acquired by Instinctif, before moving to establish the Dublin office in 2013.
She will now become senior managing partner and join the group management board.
Smith founded the public policy practice when he joined the firm in 2006 from Citigate Public Affairs, where he was chairman and CEO.
Prior to the public affairs industry, Smith worked in the UK government service for 16 years as a policy adviser, ministerial aide, and international negotiator.
Instinctif's public policy team works across Europe and the Middle East, handling clients across a range of sectors including healthcare, financial services and technology.
Instinctif group MD Tim Linacre said: "Lucy has grown the Dublin office from scratch to a team working with some of the largest businesses in the world, and now brings her expertise to the wider public policy team based in London, Brussels, Berlin, Dubai, Johannesburg and Hong Kong.
"My deep thanks go to Warwick for all of his efforts on behalf of the firm. I am delighted that he will take on the role of senior adviser, and continue to advise a select handful of clients, the public policy practice, the firm and me – I am particularly pleased that I will continue to benefit from his wise counsel."
The news follows a number of senior people moves at the City of London-headquartered agency. Last November CEO Richard Nichols moved to the new position of deputy chairman following Linacre's promotion to group MD from senior managing partner.
Instinctif announced the promotion of two managing partners last month. In May, PRWeek reported that Instinctif partner Giles Stewart had left the agency to join Buchanan to build its financial services offer.
Instinctif last month launched a new offering focused on environmental and social impact.