transfer time thru Heathrow

• Is it the same terminal, or a different terminal?
• Is this one through booking, or two separate bookings you have made yourself?

Heathrow is really 3 different airports, sharing the same runways. Terminals 1, 2 and 3 in the middle, terminal 4 on the south side of the airport, and terminal 5 on the west side.

If you have booked this as one through ticket, the airline would not have allowed the booking if they didn't think it was do-able. And, if you get delayed because the first flight is late, that is their problem, and worst case they have to put you on the next available flight at their cost. Relax.

If you have done this yourself as two separate bookings, now is the time to panic. 90 minutes is do-able in the same terminal, so long as your first flight is on time and your luggage is labelled through to your final destination (or your are hand luggage only). Worst case, you are flying from Cork to Heathrow on a budget airline, which does not allow through bookings, in which case you need to check in again at Heathrow for your second flight (2 hours before departure), and/or you need an inter-terminal transfer transfer, which eats up another 60-90 minutes minimum. Be really afraid!

http://www.heathrow.com/flight-connections

All I can say is that last spring connected thru Heathrow (athens-Heathrow- chicago) all on one ticket and we had to run RUN and was held up by their awful second security that was flirting with the girls ahead of us. Then we RAN and they were holding our flight, because it was one ticket. I think we had 90 min. and we used all of that to get thru that airport. I will not connect thru Heathrow again unless I have over 2 hrs. But if you are flying on one ticket, then they will help you with that connection. Just too much stress for me.

In my view, you do not have enough time to do things reliably and in comfort.
If your luggage is not checked through, unless you limit yourself to carry on only I don't think it's a good idea.

Presuming we are talking about a single booking, 90 mins is only advisable without a terminal change otherwise 2 hours is advisable.

Can't give more detailed advice without full details. However, l would guess the Cork flight is into Terminal 2, so the easiest connection would be onto United.