Yes, it would seem pretty likely that the Eliza Winterscale who was born in c1790 and died in Sligo in late 1880 would be the spinster sister "of Sligo" mentioned in John Winterscale's will written in 1850. As you say, there is probably little point in getting the death certificate, since Irish certificates are the same as English ones and would not give any information about parentage.
On the question of James Winterscale, the father of Eliza and John (and Mary Winterscale/Davis or Davys) I see from the Google page you found that the date of the parliamentary debate was February 1838, so that I think pins down the date of James' death to between 1838 and 1850 (when there was no mention of his father in John Winterscale's will, only of his mother. Now, looking at the IFHF site you referred to I see thay have an index record of a James Winterscale died in County Sligo in 1843, and I think this could well be him. This is the church record of the death and, if he was a person of some standing in the community, it could be that it would provide more information than just his name. I am debating whether to cough up 5 euros to have a look at that one. Incidentally, that site seems not to have any record of the 1880 death of Mary you found on the IGI.