The UK and Ireland have established the so-called Common Travel Area, meaning there are no border checks between the UK and the Republic of Ireland and the UK Border Force will assess your eligibility to enter this area. This may complicate things in your case as even though you are technically "transiting" in the UK, you might have to be landed in Ireland; which would mean you need a visitor visa. Indian nationals do not require or qualify for Transit Visas in Ireland. On the other hand, you do only have a short connection so you won't be encountering Irish Border controls which could still qualify you as a transfer passenger. It would all depend on what the border guard in the UK establishes.As from the available information, it seems I won't need Transit Visa at London while going (exempt due to Valid Canada Visa), will i need one for Dublin while going and London (since i won't have PR Card by then). Please help.
You should check with the airline, a travel agent, or the Irish and British High commission for a more definitive answer. Please do report back as I am curious to find out what the requirement is.