I am Lebanese Citizen and am planning a trip to Ireland this month. A valid Irish visa is already in hand and I am willing to reach there through the UK.
Will I need a transit visa in the UK for few hours for flight connections to Ireland?
Transit visas are not valid for transit to the Republic of Ireland (here and here). This is because, due to the CTA, you must pass through border control and enter UK territory to board the flight (I am not sure why the Visitor in Transit Visa does not apply, as it is specifically for those who need to pass through Border Control). You have two options:
The GOV.UK website says:
You’ll usually need to apply for a UK standard visitor visa.
You might be eligible for ‘transit without visa’ if [...] [you] have an Irish biometric visa (marked ‘BC’ or ‘BC BIVS’ in the ‘Remarks’ section) and an onward flight ticket to the Republic of Ireland.
Furthermore, Timatic, the database used by airlines, says:
Passengers may make a landside transit if holding a valid biometric visa issued by Ireland (Rep.) endorsed "BC" or "BC BIVS" and traveling to Ireland (Rep.)
So if your Irish visa is not biometric, you need a UK Standard visitor visa (and not merely a transit visa). This is because although you will clear Irish immigration as well on arrival, from the UK's side you haven't "completely" left the country, due to the semi-open border agreement called the Common Travel Area (CTA).