judging from the date of birth of last child to couple + date of marriage you have at least four contenders in Patrick for Ann
Pat 17820509 d/o JAMES GELL MARJERY -[?gell]
Pat 17820616 THOS GELL MARGT CLERK
Pat 17830605 JOHN GELL MARGT CARRAN
Pat 17880324 JOHN GELL ISABELL KELLY

Looking at wills of putative parents might help - the parish reg should have 'by consent' for the last but don't always rely on this.
There are a couple of others from Rushen but IMO less likely.

There are at least 28 John Shimmins born 1770-1790 - those born in German are IMO more likely - it may be worthwhile looking at any witnesses to marriage for hints of brothers etc