Hello Em
I dug around a bit; retracing your steps. The Madigan/Sheehy marrigage was August, not April. You can view the film of the 1845 parish register online via the NLI.
Thomas Madigan married Catherine Sheehy on the 6th of April August 1845 in
Ballyhahill Townland – Parish Shanagolden
Here's a few suggestions. Reading the marriage entry, we see 'residence' is given as townland called "Ballyhahill" within the RC Parish called Shanagolden. This suggests the did not live in Shanagolden village/town, but in a rural area. Hence, it is likely you can find via Griffiths Valuation the names of all the Occupiers (Tenants) of the houses and farmland within Ballyhahill.
These days you can view the Griffiths old maps and merge (overlay) them with modern (Google-style) maps. This is a good way the see where the townland is in relation to today's landmarks of highways, airports and shopping centres.
See: https://www.townlands.ie/limerick/shanid/kilmoylan/loghill/ballyhahill/
I am sure the good people at IrelandXO will be able to walk you through Griffiths Valuations and Maps.
Similarly, the child Michael (1846-1935) baptised (15Apr1846 townland Creaves; Kilcolman Parish) shows the 'residence' at the time of this birth. The word "Creaves" is my reading of the Parish Priest's writing and should be reviewed.
I could find a Dan Madigan married Mary Nunan in 1913, Newcastle District, Co. Limerick. A check into the 1901 and 1911 census could offer more information. I saw a c. 1925 death of a Dan Madigan in same district too. If this is the one and he & Mary Nuam had children you should be able to pick then up.
Similarly, perhaps one or both brothers followed Michael into Victoria, or another state in Australia. Have you searched the Australian newspapers, digitised by the NLA and available online (free) via Trove? A treasure-trove of information for useful snippets of data.
Good luck and best wishes,
Eire2Go