I think it was the No1 that saw predominant use in North Africa , I may be wrong . The No4 didn't really show up in numbers until about 41 , others will no doubt know more than me .
I know nothing about ammo availability in Ireland , but there are a lot of different brands of ammo out there . The Lee Enfield is probably the most iconic Canadian rifles I can think of , anyone who shoots has at least one or more , so ammo for the 303Brit is available everywhere here , but that doesn't help you .
As to determining if it's a shooter , the same rules apply to LE's as any rifle . Check bore condition , check serial numbers to see if the bolt and receiver numbers match , many don't , and if possible , have the headspace checked by a gunsmith as you would any rifle that seen a lot of use . They are an extremely tough rifle and will shrug off abuse that would destroy most commercial firearms . You should go online and have a look at some sites like Milsurps.com , an amazing source of good , and accurate information , I'll see you there lol . Welcome to the club .
AB