The graveyard of St Martin's-in-the-Bull Ring was extremely full by the late 18th century and was extended. William Hutton wrote in 1783:
From the eminence upon which the High-street stands, proceeds a steep, and regular descent into Moor-street, Digbeth, down Spiceal-street, Lee's-lane, and Worcester-street. This descent is broken only by the church-yard; which, through a long course of internment, for ages, is augmented into a considerable hill, chiefly composed of the refuse of life. We may, therefore, safely remark, in this place, the dead are raised up. Nor shall we be surprised at the rapid growth of the hill, when we consider this little point of land was alone that hungry grave which devoured the whole inhabitants, during the long ages of existence, till the year 1715, when St. Philip's was opened. The curious observer will easily discover, the fabric has lost that symmetry which should ever attend architecture, by the growth of the soil about it, causing a low appearance in the building, so that instead of the church burying the dead, the dead would, in time, have buried the church.
St Martin's graveyard closed in 1848 when Park Street Burial Ground opened (except for burials in family graves which continued until 1915).
Park Street was certainly affected by the cutting of the railway here. I guess the disinterred were reinterred at Birmingham Cemetery in Witton which opened in 1860.
I'm not sure when the burial ground was landscaped as Park Street Gardens, but it was revamped in 2000.
The remaining internees are soon likely to be disturbed by the building of the new station for the high-speed rail link from London which is planned to cut through the site.