There will, most likely, not be problems when arriving in Heathrow, as your luggage will be send on and the UK immigration and border control will not check it.
Arriving in India it is very likely you will run into problems, but that will depend on the seeds and saplings, their health certificates and whether these seeds and plants are allowed into India.
To make sure the best way is to buy from a good nursery or shop which can give you a 'clean seeds' health certificate for the seed and saplings, and one that is accepted in India. (I am not sure they are around, but if not, it is even harder to import them.)
When arriving in India, declare them to the customs officer or the officer who checks for health and such problems. The customs officer should be able to point you to the right person.
Transporting seeds in luggage is not a problem. Seed is quite sturdy and well packed it can survive a lot more than a flight.
Transporting plants, of whatever age, is harder and saplings are quite tender and may not survive the travel.
It does of course depends on the actual saplings and how you pack them.
Links with more information, from the comment (on the question) by @Traveller.
Ireland to UK.