It seems likely that the original "Pwll Mouric" was, infact, the inlet now called St.Pierre's Pill, which has a much more restricted reach than it once had. The first Ordnance Survey maps from the early 1800's show that the inlet essentially went back to Mathern (Map). Prior to this there had been some landscaping of the area (the large lake at St. Pierre House), however, the tidal zone really became restricted after the coastal railway was built (1886 Map), and an arm of the original creek was essentially detached (1886 Map : Closer - the arm may or may not have been altered originally to provide a harbour). In the deeper past, comparison with other pills along the Severn (See Wonder 6, for example), settlement sites, and the geological maps for the area suggests the estuary went further up some inlets than is recorded on maps. Estate maps and surveys from the 16 and 17thC suggest that at least part of the land was reclaimed and/or marsh by then*, though even now the area of the old inlet is marshy and requires considerable drainage.
The inlet takes its current name from the 12th C. knight and local landowner Roger de St Pierre*. There is, however, reasonable evidence that the name of the majority of the inlet was Pull Muric before this. from the Book of Llan Dâv (see Other Works). This was compiled from the 12th C. onwards from current and older materials and includes a number of descriptions of ecclesiastical boundaries with details professing to be from the time of Mouric, including boundaries running along the Pull Muric that make it explicit that this ran down to the Severn.
Translations of these given by Evans* include the following description of the area given by Meurig to the church of Llan Dâv after his father's death:
From the mouth of the Pool Merrick brook upwards towards the pool, to Lybiaw's stone. From the Pool to the stone to the head of the higher ground to the well of Elichguið, upwards along the brook to the well of Crug Llewyrn. From the well of Crug Llewyrn near to the mouth of Nant Bywguan. Along Nant Bywguan... From the Kiln downwards along the dyke as far as the Carn of Perth yr Onn, downwards to the ruins, to the pool. Along the pool to the mouth of the Hunger Pill on the Wye. Along the Wye and the Severn, with its wears and its landing-places for ships, as far as the mouth of the Merrick brook.
However, the words in bold are in interpolations by Evans suggesting places. The Meurig brook (now "Mounton brook")* does run into the inlet, however the text itself simply reads "Finis Oaper pull Muric [in margin: pul Muryc] sursum yrpull" and "yoggou betaper muric [in margin: vet aber meuruc]".
A survey of 1578 to determine the havens and creeks of the County of Newport lists Pill Merrick, St.Pierre and Mathern separately*, which may suggest there was an intermediate stage in the development of the area when there were three separate names and harbours recognised within the inlet, before it took the name St.Pierre's Pill.
The likelihood that the inlet is the Pwll is supported by the name of such inlets along the Severn, which are known generally as Pills. This appears to be a corruption of Pwll suggesting the term could refer to creeks as well as inland pools (though c.f. Rippon* in which it is suggested Pool is a corruption of Pill, which is suggested as meaning "Stream": both seem less likely). For example, for the above translated location "Hunger Pill" the original has pull neuynn (i.e. in modern Welsh Pwll Newyn, "Hunger Pool"). Although Davies* suggests the old name of St. Pierre's Pill was Porthiscoed ("harbour below the woods"), wherehence Portskewett, evidence for this is far less forthcoming and it doesn't seem likely Portskewett was associated with the Meurig brook.
Jones*, while discussing St.Tewdric's well, gives the location of "Pwll Meurig", as being "a short distance to the west" of the well (which Pwllmeyric village certainly isn't). This may be the pool referred to by the local in this Note, however, it seems unlikely that the inlet is being discussed as this would have pre-dated Jones and the local described the site as a pool.
There are two pools in Mathern: one is associated with a Spring 500m east of the village and is now on the edge of an industrial estate. While it is shown on the 1886 map, and therefore older than the estate, it may also be associated with drainage ditching of an indeterminate age. Had the Severn risen as far as this pool at this time (and it did at some time*), it is unlikely (but still possible) Hunger Pill would have existed, which causes some problems if we believe the Llan Dâv boundary (above) to have been laid down, at least geographically, by Meurig (it seems unlikely "Pull Muric" as a name would have existed before he gave the area to Llan Dâv).
The second pool is just west of the church in the grounds of the Palace of the Bishops of Llan Dâv (see Visit Details). This pond may or may not be shown on the 1886 map - though there is something there, it is under writing and hard to discern. It is possible that the feature referred to by Jones and the local is this pond. The fact that the remains of the palace of the Bishops of Llan Dâv (on the land granted to them by Meurig) are in the same location doesn't negate the pond being older, but there is no evidence for this. Equally, there is no evidence the pond carried a name of any sort, and the evidence more strongly points to the inlet being Pwll Mouric.