Welcome back to Rootschat. Hope things are better now and you'll be able to get some information here.
Marriage certificate for William Forrest and Margaret Russell will give exact date and place of marriage and should list both father's and their occupations as well as the names of 2 witnesses and residences of bridge and goom at the time of the marriage.
1901 census will show whether or not William was still alive as well as names and ages of all the household. National Archives, Dublin is gradually putting 1911 and 1901 census online but they only started with 1911 Dublin in Dec.2007 and it'll probably be a good while for anything else to be added.
If Margaret died in Australia you should perhaps start with her death certificate to see if you can get her age, birthplace, parents' names, etc.
www.findmypast.com should list Margaret and children leaving UK.