IGI has eight children born in Patrick to Philip Shimmin & Catherine Quine. Christening dates are as follows:
Sarah Sept. 23 1787
Ann Sept. 12 1789
Philip Feb. 5 1792 (d Feb. 15 1792)
Thomas Nov. 23 1793
John Oct.2 1796
Catherine July 14 1799
Margaret abt May 30, 1802 (extracted from parish record!)
William Jan. 1 1804
Chances are that she was born about the same time as her husband and was buried in Patrick later than Jan 1 1804.
Brian Lawsons site gives the following possible burials for Catherine Shimmin:
April 16, 1804
Dec. 1 1806
Apr 7 1809
July 26 1824
One clue is that on January 26 1806 a Philip Shimmin married Elizabeth Cowle in Patrick.
Also there is a 1803 will in Patrick for a Catherine Shimmin (film 0106408 per Brian), you may want to look at.