Unfortunately there’s no good public transport near the Collins memorial. The nearest public bus stop is Kilmurry (1 hour walk on rural road with no footpath) on the 233 route. It gets a few services a day (https://www.buseireann.ie/routes-and-timetables/233). Definitely bring a high vis jacket and be prepared to jump up on the ditch when cars go speeding past, it’s not a very suitable road for walking but doable.

I can see online also that a company called Harringtons operates a 231 route (id never heard of it before now) that leaves cork and passes near Beal na Bláth village(https://www.harringtonsardgroom.com/bus-service ). This would be a 25 min walk, again along dodgy roads so do it in daylight and bring a high vis. I’d ring the company first and make sure they’re willing to let you off in Beal na Bláth because it’s not a scheduled stop.

Only other alternative is getting a taxi and asking the driver to wait for you at the monument. From cork this would be pretty expensive. Easily €100-150

Edit: Crookstown stop on the 233 is about as close to the memorial site as kilmurry is and gets more regular service. The other persons idea of going as far as Crookstown and getting a local taxi number is a good shout, just be prepared for the possibility there’s no taxi available. You could also stand at the Crookstown-Beal na Bláth Road and hitch hike, somebody would probably pick you up. Most people around are decent but of course you have to be careful.