The two raiders targeted the Credit Union in Portmarnock, Co Dublin. After terrorising staff, the men were confronted by the garda and fled onto Strand Rd.
They fired shots, hijacked a car, and escaped with cash taken from the heist.
The incident happened at about 5.30pm. The credit union was open late for Friday business.
During follow-up searches in the Belmayne estate, approximately 4km away, gardaí arrested one man in his 20s. They also recovered cash and a handgun.
The man was detained overnight at Coolock Garda Station under Section 30 of the Offences Against the State Act.
Two scenes were sealed off for technical investigations.
In a separate raid in Phibsboro, also on the northside of the city, two men held up tellers while wielding knives.
The men demanded cash after arriving into the institution at 4.25pm.
They threatened staff, took money and left on foot along the Phibsborough Road.
However, within a short space of time, gardaí on their way to the scene picked up the suspects.
The stolen money was recovered and they were taken to Mountjoy Garda Station for questioning.
Investigations into both incidents are ongoing.