When and where did this couple marry?
In all the newspaper accounts of the death of this couple, 1867 - What names do you see in the newspaper articles for the children? What ages do you see in the newspaper articles for the children? What do you see in the newspaper articles to indicate the number of children left as orphans at 1867?
The newspaper articles don't give more details about the kids - I've attached an article to this, and another's quoted at the end.
And I don't know when they married. It was before civil registration and the marriages in the parish records for Castleconnell start in 1863. I'm basing it off the fact that Thomas was 38 and Catherine was 34 when they died in 1867. I'm fairly certain they married in Castleconnell parish because Thomas's brothers also lived there, and there's no marriage for a Catherine Hanifan and a T Fitzpatrick in all of Ireland during this period. I've checked using RootsIreland, which has a comprehensive index of the parish records.
The news item I think you are mentioning here was for the death of a man in 1937 in Melbourne.
https://trove.nla.gov.au/newspaper/article/11111832
Sue
Thanks for the correction - that's what I get for posting right before bed!
Just to discount him further, trying to work out whether this Martin FITZPATRICK (who died 1937) lived alone: he lived at 37 Tribe St, South Melbourne (according to the probate advertisement). At the same address in 1940 a James FITZPATRICK died.
https://trove.nla.gov.au/newspaper/article/204391169
No mention of a Martin (deceased) among the family mentioned in this death notice.
A coincidence of names, or are they family?
Modified to add:
This death notice for Catherine FITZPATRICK in 1934, of 37 Tribe St South Melbourne, doesn't have a mention of a Martin either.
https://trove.nla.gov.au/newspaper/article/204811250
But it seems there were a family of FITZPATRICKs at the address, so I think you can assume the Martin who died there in 1937 was probably not yours. 
That's some great detective work, thank you so much!
so the will extract I posted is not your guy?
I think the one you posted is the most likely to be him - I wonder if Nellie Walsh might have been the lady he was "boarding" with (delighted to know I'm not the only one suspicious of that!). My local library has access to FindMyPast, so I'll see if it has access to the Australian probate stuff too.
The Garvey connection is also quite interesting - having a cursory look at Castleconnell parish, a Catherine Garvey is baptised there to Thadeus Garvey and Ellen O'Donoghue in 1864. And the most likely option for Rev. Garvey himself is a James Garvey born in Hospital & Herbertstown parish in 1841.
James has two brothers named Timothy, and there's a marriage between Timothy Garvey and Ellen Donoghue in Limerick city itself in 1854. They apparently lived there until Catherine's birth and his name varies between Thady and Timothy. So it looks like Rev. James Joseph Garvey had relatives locally.
(Later edit: One obituary explicitly says he was born in Herbertstown parish and he went to the same school as John, so that's likely the connection -
https://trove.nla.gov.au/newspaper/article/115468122)
And here's that article from the Cork Advertiser, 12th January 1867:
A MAN AND HIS WIFE DROWNED
Limerick, Monday 7th Jan. - A very painful sensation was created yesterday in the vicinity of the Canal Harbour, off Irishtown, in consequence of the bodies of a man and a woman having been found drowned in the basin at the Limerick side of the City of Dublin Company's Stores. Shortly after 7 o'clock in the morning a donkey, with car attached, were discovered in the water near to one of the flood-gates, the animal quite dead; and from a noise being heard about 8 o'clock on Saturday evening, as if a person had been drowning, it was conjectured that the body was close by.
The police of George's-quary Station, under Constable Shea proceeded to the scene, and a boat and grapplings having been procured, men proceeded to drag the bed of the canal. [...] It turned out to be that of a female named Catherine Fitzpatrick, of Knockbrack, near Annacotty, wife of Timothy Fitzpatrick, a respectable shoemaker, who, it was then ascertained, had also been missing, both having come into town, a distance of three miles and a half, upon a donkey's car on Little Christmas Eve [January 5th], to purchase necessaries for themselves and their children.
The search having been continued, the body of Timothy Fitzpatrick, the husband, was picked up in about an hour after that of his wife had been discovered. [...] The sad occurrence attracted crowds to the canal, and the relatives and friends of the unfortunate couple having come in from the country to look after them, the scene which took place was heartrending. [...] What renders this melancholy occurrence still more painful is, that the woman was enceinte [heavily pregnant], and known to be within a fortnight of her confinement. She was aged 34 years, and her husband 38.
