I'll jump in as the token American here who grew up with firearms and whose elder brother served for 10 years in the infantry...
You are both partially correct. Double barrel rifles do exist, and are used for big game hunting, but they lack long range accuracy, are a huge caliber, and are not designed for any sort of military application. IF one is going to use a double barrel rifle for a military application, one should use an over/under (where the barrels are stacked upon one another), rather than a side by side, because the degredation of accuracy there is will be vertical rather than horizontal, and people are longer vertically. And, yes, these sort of rifles are better at close to mid range.
Here in the States, though, shotguns loaded with shot (a bunch of pellets, what one typically thinks of for a shotgun) are standard for hunting small game (birds, rabbits, etc.), and loaded with slugs (basically a big bullet) are standard for larger game (bear). Mid sized game is typically hunted with a .22 caliber rifle, which has a significant gain in range and accuracy over shotguns or double barreled rifles, and also almost no recoil. These are typically either bolt action (you have to pull a lever back and then push it forward again to load another bullet) or semi-automatic (the recoil loads another bullet, so it will fire more or less as quickly as you can pull the trigger, but it will jam if you pull it too fast). Bullets of this sort have a hard time penetrating bone, especially at range, so to use them in a military application would require shooting for soft tissue. But, being many of these have 10 round magazines, they would be preferable over either shotguns or double barreled rifles, which can fire once or twice before reloading. Unless, of course, you are talking a pump action shotgun, which often hold 8 shells...
Now, all this information assumes that your protagonist's society looted an American sporting goods store. If they looted a European one, their available armaments may be more reduced or greater, depending on where you are and how shady the sporting goods store is. I am assuming lesser in Ireland, unless they happened to have a stockpile of illegal firearms. Now, if they loot a police armory... whole different ballgame. Being we are talking about Ireland, I suspect they would have access to 5.56×45 NATO rounds with M-16/AR-15 variants (30 round magazine, semi-automatic and 3 round burst fire modes, effective to around 800m in skilled hands) and precision rifles (bolt action or semi-automatic, usually 10 round, effective to around 1km in skilled hands with a scope). If it were a military armory, the sky is the limit... automatic shotguns with drum magazines (excess of 50 shells), machine guns (fires for as long as you hold the trigger), sniper rifles effective up to 3km that can cut through light armor...
All this to say, the sort of firearm your protagonist uses will depend greatly on the backstory, especially where the faction gets its weapons.
Now, on the tactical side of things, a rifle can be effective in close quarters, but it really depends on the setting. If you are in open field, sure, it isn't much different from any other full sized gun. But, inside a building and in urban combat, the length of the rifle becomes a big disadvantage, because you cannot turn as easily in tight spaces. For these settings, a carbine, bullpup rifle, or sub-machine gun are vastly preferable, because they are a good bit shorter than a full rifle. Pistols are also pretty useful in this context, because they are very small and their main drawback (poor ranged accuracy) is negated with the close combat. I'll also say that if you are using a bayonet or the stock of a rifle against an enemy, something has gone really bad, really quickly. The USMC almost never uses bayonets at this point, preferring knife fighting and an evil cross of Ju-jitsu and Krav Maga. Given my preference, I would take a saber over most any other weapon at that range, and things like that should be accessible to your protagonist...
Anyway, I hope all that is helpful. This was intended as a broad overview, so feel free to ask if you have any other questions specific to a situation.