There is this marriage in St Helens which might be your Margaret?
7 Oct 1882 Margaretam FITZPATRICK Michaelem ROCK St Mary's (Lowe House) RC
December Qtr 1882 Prescot 8b 1094
Having said that, the only Margaret Rock married to a Michael that I can find in the 1891c is at 31 Atherton St, Windle, St Helens and the record has been transcribed as 'Roch'. Unfortunately, it shows that this Margaret is 29 and from Queens County, Ireland. So doesn't look like yours anyway.
I can't see a death for Margaret Rock between 1882-1891.
Will keep looking.
Kit