The town of St Marys is physically dominated by the mountain of St Patricks Head, named from the sea by Captain Tobias Furneaux on St Patricks Day ,
17 March,1773, 30 years before European settlement of Van Diemans Land, as he sailed past in the "Adventure" on his way to New Zealand after having been separated from Captain Cook.
The significant dates in the development of St Marys as a town name are 1842, 1858 and 1864. Prior to the 1840's the area was generally known as the St Patricks Head District,but then the property of "Cullenswood" developed as a de facto township,until
in the 1860's St. Marys took over as the capital of the Break-O'Day Plains. This development is reflected in the official names of the post office for the area,
St Patricks Head from 1 June 1835, renamed

Cullenswood 23 April 1845 until the St Marys Post Office opened 11 April 1869.
St Marys did not in fact exist as a name in the area until 1842. A convict probation station was established midway between the present day town of St. Marys and the top of the Pass, in order to build a Pass over the Tiers to the coast at Falmouth, to provide an adequate and permanent route from the inland farm properties of the area to the Port of Falmouth. The site of the station was called Grassy Bottom.

A "prime mover and shaker" for the construction of the Pass had been a Scot, James Grant of "Tullochgorum". While delighted that the construction was to proceed, the local "establishment" was less than pleased with the prospect of "Grassy Bottom Pass" appearing on the map. Grant wrote to the Surveyor General in June 1842, and proposed that the Pass be called "St. Marys Pass (or Vale), Falmouth.
"A second Probation party has been determined on by His Excellency Sir John Franklin.....to be stationed at a place hitherto known as the "Grassy Bottom", a pass on the tier adjoining that of St. Patrick's Head and within about four to six miles of Falmouth - of course the site selected on the intended line of road and as the vale will now come much more into notice, and the name being anything but euphonious, it has been agreed on all hands to rechristen it, and I have proposed to the Director of the Probation Department "St. Mary's Pass or Vale, Falmouth"...."
Why "all hands" wanted it rechristened specifically as St. Marys Pass, as opposed to (say) St. Patricks Pass, is unknown. The Pass was completed 1843-1845 and opened in 1846 as St. Marys Pass.

From then, the areas was referred to both as St. Patrick's Head and as St. Marys Pass. An early reference to St. Marys Pass as an "abode" was in the baptism record of James Canham (son of James & Jane(Cook) at Cullenswood 4th December 1854.
But still Cullenswood was the major settlement centre, and the 1858 Valuation Roll for the Fingal District showed 54 occupiers of property on Cullenswood, Londavra and Harefield, all showing their address (abode) relative to Cullenswood, eg. "Londavra near Cullenswood", "Harefield near Cullenswood" etc. In the whole Roll, only 3 entries refer to "St. Marys".

Nevertheless, it seems that the name St. Marys had emerged as a place name by this time, as short for "near St. Marys Pass". More significantly a supplement to the Valuation Roll showed 2 entries specifically referring to St. Marys as a "township"; Elizabeth Handley owner and occupier of a dwelling house and premises on 2 rods 35 perches, and John Press in a dwelling house on 3 acres of land owned by Groom of Harefield, Harefield both located on "St. Marys Township". Also 3 further entries showed Harefield as "near St. Marys" rather than "near Cullenswood".

In 1858, for the first time, St. Marys appeared on a map; Wellers Map of 1858 shows not only St. Marys Pass, but also St. Marys. However the legend to the map does identify it as "a town not yet settled".
However, official recognition was soon to follow, and the "Township of St. Marys" was proclaimed in the Hobart Town Gazette on 26 April 1864, by power of Section Two of the Waste Lands Act 1863! Walch's Almanac for 1864 described the new township as: "132 miles from Hobart 78 miles from Launceston; population 50, 1 constable. There are about half a dozen tenements, and a post office on the township, including the house of Mr. Thomas Hardy at the side of the rivulet, which is crossed by a log bridge."
Apparently at one stage "Armagh" was considered as a name for the township, presumably to recognise the birth place in Ireland of a local worthy, Frederick Louis (von) Steiglitz of "Killymoon", and to consolidate the Irish flavour of Avoca and Fingal. However, he returned to Ireland in 1859 so this lapsed. So, St. Marys it remains to this day.