Some of the weapons were discovered when officers stopped two vehicles in Ratoath - the same village where a man was shot dead on Wednesday night.
Two assault rifles and ammunition were found in the vehicles and in follow-up searches nearby two pistols, silencers and more ammunition was recovered.
Police said the operation is ongoing.
It involved officers from the Drugs and Organised Crime Bureau, supported by national police units, including the Emergency Response Unit.
The investigation follows the fatal shooting of Noel Duggan in The Old Mill housing estate in Ratoath at about 19:45 local time on Wednesday.
Mr Duggan was a well-known criminal linked to cigarette smuggling and also had connections to a Dublin family who were caught up in a fatal feud in the city last month.
A police statement said Thursday evening's operation was targeted at "organised criminal groups involved in the provision and distribution of firearms".
The men who were arrested are in their 30s.
Three men have been arrested after police seized guns and several hundred rounds of ammunition in County Meath in the Republic of Ireland.