Nine people have been arrested as part of a cross-border police operation targeting the Irish National Liberation Army (INLA).
The PSNI carried out eight searches and arrested eight men and a woman.
Police say the operation, which took place in the last 48 hours, will have a significant impact on the INLA's drug-dealing activities.
Searches in Northern Ireland were carried out in the Galliagh area of Londonderry and Limavady.
Gardaí (Irish police) also carried out five searches in Donegal and two in Dublin.
The PSNI said suspected Class A and Class B drugs, as well as a significant amount of cash, were seized.
Those arrested were aged between 27 and 42. The woman has been released on bail, while the men remain in custody.
'All forms of criminality'
Det Ch Supt John McVea said: "The INLA in the north west are involved in all forms of criminality including the organised illegal supply of controlled drugs.
"This operation looks to take action against those involved in this illegal drugs supply.
"These criminals don't care about the wider impact their drug dealing will have on the wider community and public services."
He added: "This operation is a prime example of how the Joint Agency Task Force (JATF) works on the ground to tackle organised and cross jurisdictional crime.
"The Cross Border JATF continues to provide opportunities for, and promote real time collaboration, between An Garda Síochána and the PSNI in our combined efforts to frustrate, disrupt and dismantle the activity of organised crime groups."
Garda Supt Goretti Sheridan said the operation had been ongoing since the summer of 2020.
"An Garda Síochána and the PSNI have been liaising and gathering intelligence regarding the unlawful activities of the INLA, an unlawful organisation, and their involvement in the sale and supply of drugs in the north western part of the country on both sides of the border," she said.