Is it easy to get a train from Lisboa to Seville? How long does the journey take and is the train run daily and is it frequent? Thank you all in advance.

No direct trains. Either go via Madrid (basically only the overnight train goes direct; in day you must change three times with dicey connections. Or go via the Algarve (train to Faro and to border at Villa Reale (?) and then take ferry across river to Spanish town then bus to Huelva where you can get a train to Seville. Better: Train to Faro from Lisboa and then take the buses that go direct from there to Seville. will take all of a day if possible,i havn't checked schedules. Overnight train to Madrid and then AVE to Seville may be quickest route.

If you're in a hurry, you might be better off flying. Check out iberia.com and maybe whichbudget.com

How about by bus/coach? How long does it take and does it run daily and has a frequent schedule?

Depending on how much time you have, flying would be the quickest way, but taking the bus would be the least expensive. Driving takes about 5 plus hours, so I would assume that the bus would take a little longer since there is no direct run. You will have to go through Badajoz before heading to Sevilla.

There is a (Eurolines) bus that runs about 3-4 days weekly Lisboa-Sevilla: It takes about 9-hours (incl. 1-hour stop for dinner). Last time I took it Viajes Juliá was the operator.

Or, you could take train to Faro and bus (Damas S.A.) to Huelva or Sevilla.
Check the Eurolines and DamasSA web sites for schedules & tariffs.

Depending on where else you're going it may be simples to drive. We've done it twice - and you can do it easily in one day with a couple of stop-offs on the way: Merida for the ruins or ???.