When and to what extent should the money supply of a pegged currency be included in the "parent" currency?
During the Irish Pound-Sterling pegging era, we again had separate monetary authorities, but currency union. (...) In this case GBP was mostly accepted for trade in Ireland, while IEP was rarely accepted in the UK. Would GBP be in the IEP money supply, but the IEP not in the GBP?


Let the states and the institutions declare what they want. To study economic phenomena we must look at what economies actually do. Think of something totally unrelated: each state has a specific tax structure, a range of various taxes, progressive, proportional, whatever. The official structure matters, because it anchors the effective taxation, but effective taxation itself matters more.
In your case, the Irish Economy incorporated in its workings the official monetary union, while the UK Economy didn't.

So one has to go per case, which creates a testable hypothesis: 

a) postulate an hypothesis based on "casual" (i.e. true but unquantified) observations like the 2nd sentence in the above quote
b) derive the theoretical implications of the hypothesis
c) send your theory and your theoretical results out to meet the data.  

For your example, assume that UK increased steadily its money supply to accommodate a growing economy, while the Irish economy was stable (or stagnant, depending on your philosophical point of view), or grew at a lower rate. If we add the GBP quantity in the money supply variable of the Irish Economy as equivalent, $1:1$ to the IEP, shouldn't we observe excessive inflation? We should. Did we?  Let's say "partly". Then we can estimate the effective ratio between the two currencies as regards their effect on the Irish Economy.