Brewin Dolphin has agreed to pay £37.3m to buy Investec Wealth's Irish business.
To fund the acquisition, Brewin Dolphin will raise £60m in new shares.
Brewin Dolphin's chief executive, David Nichol, said: "This acquisition, which is consistent with our strategy of growth in assets under management, provides us with an exciting opportunity to strengthen substantially our existing presence in the Republic of Ireland, one of Europe's fastest growing economies."
The company said it would create a "top three wealth management business" in the Republic of Ireland with assets under management and advice of more than €4.6bn (£3.9bn).
Brewin Dolphin revealed it was in talks to buy Investec Wealth's Irish arm last month after "media speculation" about a deal.
The announcement comes as Brewin Dolphin announced its results for the six months to the end of March 2019, with assets under management of £42.4bn compared with £42.8bn at the end of 2018.
The discretionary fund management business had assets of £37.5bn at the end of March, compared with £37.6bn at the end of 2018.
Brewin Dolphin’s statement said the DFM business attracted net inflows, but didn’t disclose a figure and said the growth of its assets under management had stalled during the three month period because of "investment performance".
The company made a statutory profit of £29.7m, a drop of 12 per cent when compared with the same period in 2018.
Mr Nichol said: "In the first half of 2019, the group has continued to deliver strong and resilient organic growth, even with the backdrop of volatile market conditions. This is demonstrated by the strength of our discretionary funds flows. Our strategy of focusing on our advice-led wealth management service in the direct market continues to deliver results.
"We are investing in our business to support future long-term growth. Over the past few months we have announced the replacement of our core custody and settlement system and a number of acquisitions. These initiatives are laying the foundations for long-term growth and will ensure that we are well placed to capture market opportunities."
david.thorpe@ft.com
