Hi,
I think Niall, Backslash, and Eric have the topic covered. It is important to realize that muskets from this early 18th century period were often purchased by the regimental colonel from private contractors. They had minimal specs and standards required by the government but the colonel could decide a great deal of the design and quality. As a result, the muskets varied a lot. However, because your lock is marked with the crown over AR and also the small crown over the arrow, I believe it was a direct government ordnance purchase for the Royal Irish Regiment perhaps around 1710 - 1715. The small crown over the arrow indicates government ownership. The harp engraving is the official symbol of the Royal Irish Regimant. Nonetheless, I suspect the lock and gun were made in Holland and purchased by the government for the regiment. The lock is especially well made for the time having an internal bridle.
dave