John Shimmin's age is consistent in the censuses as b.c.1795, and birthplace given as Patrick. There is an extracted entry on the IGI for JOHN SHIMIN Christening: 28 MAR 1794 Patrick, Father: THOMAS SHIMIN, Mother: MARGT CRAIN, as well as the one you noted bapt 2 OCT 1796 Patrick, Father: PHILIP (not Patrick?) SHIMMIN, Mother: CATH QUINE.

These entries are on LDS film no.0106738, as also is their marriage on 13 SEP 1823 Patrick.
Can you look at this (par reg) film at a local Family History Centre, or in the Manx Museum library? It might have additional details which will identify which Shimmin family is which (e.g. name of farm, or other place name).

There seems to be only one Elinor b.German around the right age, who is the one you asked about. The LDS film no. is 0106723 [1760 - 1856] - so it should also have their younger children's bapts. If looking in the Manx Museum the parish register films are numbered differently to the LDS ones, but have the same information.

Brian Lawson's burials index has an entry for: SHIMMIN, Eleanor 52 German 27 Oct 1855.

It might be worth asking whether there is a M.I. for her when Donna gets back, if no-one answers in the meantime.

Sue