I doubt that you'll get an inclusive package but it's not rocket science to put your own together!
Buy theatre tickets here:
https://www.the-mouse...uk/online/default.asp
A hotel just down the road, which has been frequently been praised by ABers:
http://www.strandpalacehotel.co.uk/
Reviews on TripAdvisor:
http://www.tripadviso...l-London_England.html
A couple of sites where you might get discounted bookings for that hotel:
http://www.booking.co.../strandpalace.en.html
and
http://www.laterooms....nd-palace-london.aspx
That just leaves the flights (plus transfers, of course):
From Belfast International Airport you can fly to Heathrow (Aer Lingus), Gatwick (easyJet), Luton (easyJet) or Southend (easyJet, starting from 2nd April).
From George Best Airport you can fly to Heathrow (BMI), Stansted (BmiBaby) or Gatwick (FlyBe).
From Dublin Airport you can fly to Gatwick (AerLingus & Ryanair), Heathrow (Aer Lingus & BMI), City (Air France [CityJet service]), Luton (Ryanair) or Stansted (Ryanair).
Taking some random midweek dates in mid-May, Ryanair are offering a return fare, from Dublin to Gatwick, for €45.23. That includes all taxes and charges; if you were travelling with just hand luggage, the only extra would be a €6 admin fee in each direction, making €57.23 per person in total.
A return ticket from Gatwick to City Thameslink costs £18 on the day, or you could seek cheaper 'Advance' fares here:
http://www.nationalrail.co.uk/
(You can probably walk to the Strand Palace Hotel from there. Otherwise a 23 bus, along Ludgate Hill and Fleet Street, will take you there).
Chris