The Shamrock line did exist in the 50's but production was very low and they were unbanded. That's not to say that absolutely no banded shamrocks exist from the 50's, there was likely a transition in 1950 where some of the banded pipes were still being produced and new COM's were being used, but it wouldn't have lasted for very long. Shamrock's like all other Peterson, can be dated by the COM. Do you have any banded Shamrock's that have Republic COM's? The Shamrock did for a short period have it's very own COM, A Peterson Product of Ireland. It was used in somewhere in the 48-49 time frame, but it is still Pre-Republic. After 49 all peterson Products were stamped Republic of Ireland. I've seen may Republic of Ireland Shamrocks but every one of them was un-banded.
Shamrock's can be split into two distinct categories, 1936(ish)-1949 Pre-Republic and 1950<.
The early PR Shamrocks were produced from 1936-39or40 with one set of shape numbers and very dark stain, sometime around 40 they transitioned to classic range shape numbers that we use today. Around 1947 or 8 they started using a lighter stain on them. At the beginning of the Republic period the Shamrock shape list was dramatically reduced from about 40 shapes to only a handful, they are mostly seen in X150, 107, 999, & 87 there were a few others but they are less common.