I realized that the IRA shifted and molded throughout time but at the height of its power, how many people were in and/or supported the IRA in Ireland. I can't find any hard figures.
I think it would be good to specify what particular IRA you wish to know about. Are you talking about the Old IRA that Michael Collins fought with? Or the Provisional IRA?
Martin Melaugh at the well regarded CAIN website gives a lower estimate than Coogan. According to him the IRA had about 1,500 members in the early 1970s and fewer than 500 by the time of the ceasefire. However this was partly deliberate. In the early 1970s, the IRA still tried to be a mass movement but after 1979 it reformed into a cell structure with a smaller membership. Arguably the mid 1980s were actually its strongest period as it had received huge quantities of arms from Libya. The difference in figures is impossible to resolve as the IRA was a very fluid organisation which often did not know its own full strength especially in the chaotic early years.
Popular support is very hard to gauge. As has already been stated there were substantial numbers of supporters especially in republican areas such as West Belfast or South Armagh. It was also extremely dangerous to oppose them in these areas so many people acquiesced. There were many others who opposed the IRA but opposed the British government even more strongly.
Sinn Féin received 13% of the vote in Northern Ireland in the 1983 general election with substantial support because of the hunger strikes. It lost votes at each election until the ceasefire, bottoming out at 10% in 1992. In the Republic, it hovered between 1% and 1.6%. Given that its vote increased steadily following the ceasefire of 1994, most analysts believe that public opposition to the IRA's campaign was the main reason for Sinn Féin's poor showings during the Troubles.