Matheson is now the largest law firm in Ireland, according to figures published by the Law Society Gazette.
The figures reveal Ireland's 20 largest law firms in 2020.
Matheson had been the third largest for the previous six years.
It has now overtaken the two firms that were ahead of it throughout that period, namely A&L Goodbody and Arthur Cox.
The total number of practising certificates issued by the Law Society on 31 December 2020 was 11,854.
The list, which is compiled using the number of practising certificates of the largest firms at the end of each year, is published by the Law Society of Ireland.
With 327 practising solicitors, Matheson has passed out both A&L Goodbody and Arthur Cox LLP, which held the top spots between them over the past six years.
Matheson recorded a year-on-year increase of 42 practising certificates - or 15% - over its 2019 total, which was unchanged on 2018.
It puts it seven solicitors ahead of A&L Goodbody and 15 ahead of Arthur Cox LLP.
They were the only firms with more than 300 practitioners.
McCann FitzGerald and Mason Hayes & Curran LLP make up the remainder of the top five.
Brexit effect
11,854 solicitors held practising certificates issued by the Law Society by the end of last year, a decline of 105 - or just under 1% - on the record 11,959 at end-December 2019.
This was mainly accounted for by a decline in the number of what are referred to as 'Brexit' practising certificates taken out in 2020 compared to the previous year.
This stems from a decision by the Society in the latter part of last year to no longer issue practising certificates to firms of solicitors who do not have an office, or do not plan to have an office, in this jurisdiction.
"The phenomenon of large international law firms, with no establishment in this jurisdiction, taking out Irish practising certificates for their solicitors who have recently come on the Roll here was a by-product of Brexit and has now come to an end," the report claimed.
