The general point remains that these are quite rare, appear in areas of prior known Celtic settlement (but subsequently to the "local" Celts having switched to cremation burials), and reflect "Celtic religious tradition" (J. Kostrzewski). The Celts are known to have compactly inhabited three areas of Poland: near Wroclaw, in Upper Silesia, and around and about Cracow. The Przeworsk culture inhumations were first discovered in the Wroclaw enclave. Now we have some interesting material from the upper Vistula (at Pelczyska on the Nida, near Cracow), part of the so-called Tyniec group.
Cf.
http://www.archeo.uw.edu.pl/szablon.php?id=830
Marcin Rudnicki writes as follows about Pelczyska:
"The largest number of finds from site 6 is associated with the gravefield of the Przeworsk Culture people, set up over the site of older cemeteries during the 1st c. BC. Burials from this period discovered at Pełczyska are a reflection of the complex culture situation of the period (Tyniec Group). A definite majority are cremation burials, within pits, with no traces of the pyre (‘pure’), reminiscent of grave forms of ‘Silesian’ type. The first urned graves – very few – come on record only during the second half of AD 1st c. (phase B1b). Next to ‘ordinary’ pit graves, the cemetery at Pełczyska harboured a peculiar grave deposit – a ‘ditch feature’ – on a rectangular plan, 6 by 6 m, datable to phase A3 (LT D2). This type of grave is known from other sites in the region and its presence at Pełczyska is interpreted as the result of Celtic influence on burial rite in Przeworsk Culture. Truly exceptional are two inhumation burials
– the only such deposits from the Pre-Roman Period known from this region of Poland. The complex issues of the origin of inhumation on Przeworsk Culture territory and of the origin of the woman buried in the grave at Pełczyska, are discussed in a separate publication (Rudnicki 2005)."
This separate publication is:
"A Late La Tène inhumation grave from Pełczyska: Comments on the cultural situation in the upland area of Little Poland (with analysis of anatomical remains by Karol Piasecki), [w:] H. Dobrzańska, V. Megaw, P. Poleska (red.), Celts on the margin. Studies in European cultural interaction VIIth cent. BC – I cent. AD. dedicated to Zenon Woźniak, Kraków 2005, 195-206."
I don't have access to this item here. It seems to me that Rudnicki would be arguing for Celtic influence here also, and that the incomers were from areas close to the Alpine Roman border where inhumation was still practiced among the Celts at that time (but one would obviously need to read the work).
Rudnicki's coordinates are here:
http://www.archeo.uw.edu.pl/szablon.php?id=356