This is around the period covered by Griffiths Valuation. Names of McGrottys attached here below:
Mc Grotty John Big Glebe Dunboe
Mc Grotty Michael Altikeeragh Dunboe
Mc Grotty Charles Churchland (Part of) Killowen
Mc Grotty William Churchland (Part of) Killowen
Mc Grotty Margaret Newtownlimavady Drumachose
Mc Grotty Mary E. The Diamond,Coleraine Coleraine
Mc Grotty John B. Portstewart Ballyaghran
Mc Grotty James Coleraine Coleraine
Mc Grotty Mary Coleraine and Suburbs Coleraine
Mc Grotty John B. Main St.,P'stewart Ballyaghran
These are all from the North West corner of County Derry.
I suspect McGurn is actually a contraction of a more common name such as McGarran or McGahern. Check Griffiths fully through a site such as http://www.londonderry.wordpress.com
and happy hunting!