Gardai are unarmed to break with the tradition of and to contrast to the armed RIC which is repeated a lot on Boards.
No-one thought to look at the origin of this. The civic guard were initially armed, but disarmed by the government following a mutiny.
The Civic Guard was to be a new police force for the new state which would draw on the tradition of both the (disbanded) RIC and the Republican Police (established by Dáil Éireann during the War of Independence). The problem here was that, on the one hand, the Republican Police didn't have much of a tradition — it had only been around for two years, it had minimal structure and no training, and most of its members were IRA Volunteers for whom policing duties had been an occasional sideline — and, on the other hand, the RIC was cordially hated by a large number of even the pro-Treaty IRA volunteers/supporters who were recruited into the new force.
The Provisional Government was aware of this tension but there wasn't a lot they could do about it - the RIC was pretty much the only source of policing experience and knowledge that they had to draw on. They set up a Committee to oversee the establishment of the new force which included both senior republicans (Michael Staines, Eoin O'Duffy) and senior RIC officers (District Inspectors Patrick Walsh and John Kearney).
However well-intentioned the Committee members may have been, and however committed to the establishment of a new force, that attitude did not really filter down to the ranks of the recruits. There were reports, for example, of recruits refusing to salute training officers who were ex-RIC. This wasn't helped by the fact that anti-Treaty republicans were attempting to infiltrate the new force to destabilise it.
The new force was always intended to be radically different from the RIC. While it was not initially to be wholly unarmed — the disturbed state of the country and the virtual breakdown of policing in the previous months precluded that — it was never intended to rely on the force of arms. It's notable that when the mutiny broke out at the CG training depot in the Curragh, the mutineers had to raid the armoury in order to secure weapons, which tells us that the Civic Guard had access to firearms, but they didn't routinely carry them when on duty.
The force wasn't disarmed solely in response to the mutiny. The immediate response, once the mutiny was suppressed, was to appoint Eoin O'Duffy as Commissioner (in place of Staines, who resigned) and to shift the headquarters of the force from the Curragh to Dublin Castle/Ship Street (where it would be easier to keep an eye on things). It was another three months or so before the commitment to unarmed policing was made, and that was motivated not only by the mutiny but also by a desire to break with the RIC tradition, a desire to make the new force less likely to be a target in the Civil War which had by then broken out (and this was quite successful - only one member of AGS was killed by Republican action in the Civil War) and a desire to avoid a repetition of incidents like the one on 20 September when CG Charles Eastwood was accidentally shot dead in the barracks at Ship St by CG Leo Herde. The CG was renamed An Garda Síochána in 1923, and in 1925 was merged with Políní Átha Cliath (formerly the Dublin Metropolitan Police), which had always been an unarmed force, and which was regarded much more favourably by IRA and ex-IRA men than the RIC had been. This undoubtedly helped to bed down and cement the tradition of AGS as an unarmed police force