I have a Schengen visa, multiple entry, issued by Dutch embassy, London; valid till October 2019 and i am travelling Amsterdam from 27-29 july. However, I am planning to attend a seminar in Ireland in September, which was not planned earlier and not mentioned in previous schengen visa application. Now, Do I need another Schengen visa? If yes, will this be granted within existing visa validity?
The Republic of Ireland is not part of the Schengen area - your Schengen visa is not valid there.
Depending on your citizenship you may need a new visa specifically for the Republic of Ireland.
'Ireland' could, of course, mean Northern Ireland. Northern Ireland is part of the UK and is also not part of the Schengen area. You may still need a visa but you'd apply to the UK instead of Ireland.
Note also that visas from Ireland and the UK are generally not valid to travel across the border between the two countries. There are exceptions for some nationalities depending on which way you travel.
Thanks to Traveller for picking that up in the comments.