" ... having a hard time figuring out how you'd carry and use a flintlock weapon at least 48 inches long from horseback...

This presupposes that the horse was used for more than just transportatiion. If the Rapparee is running / fighting / snipeing / whatever - he's probably not regularly firing from his horse. Fighting on horseback against supereor forces is generally foolheardy. You enemies have a good shot of hitting your horse and really messing up your mobility.

pax yall