You could contact the 2 Knagg's still in the phone book for Ireland, two in Dublin area. None Westmeath.
Ireland is hard to search at the best of times.
Griffith Valuation was a valuation of land for tax purposes
and took place in Westmeath in 1857. At that time there are
no Knaggs listed. However, along with other counties there are Knaggs in Co.Lough/Meath (Drogheda)St.Peter's Parish.
There were only 33 Knaggs in all Ireland at that time.
The Tithe Valuation was also a valuation of land for tax purposes, it took place through Ireland 1820-1839. No Knaggs in Westmeath but 14 elsewhere. Queen's County now Laois and Tipperary N. Robert and James seem to be the Knaggs first names. What are your George's and Lucy's children names, their parents names would of been used at some point.
MUllingar was a Townland, a Parish and a Union. A Union town
covered an area of 12 mi. and contained all parish's, townlands, villages in that area.
It is also a Townland in Donegal. Mullingar in Westmeath
could also be in the Union of Castletowndelvin.
Records for baptism and marriages before 1864 are Parish only. These parish records are now in a database of an Irish History Foundation for most counties. I would do a search with just the name George Knaggs and birth year for Ireland rather than a particular county. George was quite an unsual name then as was Lucy. A baptism record should have both parents names and mother's maiden name.
Were the family RC or Church of Ireland as there is a difference in where to look for other records.www.familysearch.org
has some records for Australia
has an index with links to various websites