There is a group of 4 going to Dublin in July. One person will not know until the end of April whether or not they have gotten a job they are applying for. Is that too late to purchase our airfares? We do want to purchase the lowest cost tickets as possible. We are concerned that if we buy three now and the fourth later that there will not be even one more available.
That is close to an impossible question with no absolute answer. Airline ticket pricing changes nearly daily and many times for reasons only known by the computer. And you cannot guarantee that the tickets your purchase will be the lowest costs either. It is a dice roll. You buy the tickets when the price is acceptable and move on. There were will be tickets available in April for your friend. And you friend might even find a ticket cheaper than your tickets but may have to fly a different route and met you there. Tickets in July will not be the cheapest since this is peak tourist season. If helps a lot to fly from a major city like New York or Chicago. Good luck.
To add to Frank's excellent answer, airlines now use computers to adjust prices multiple times a day. So, no one can predict what prices will be on any given day.
Where are you flying from, and how much flexibility in routing do you have? It is unlikely, but still possible, that a particular plane will be fully sold out. In that case, your "fourth" will have to take an alternate route. If you're coming from a place with multiple options (meaning you have multiple nonstops to Dublin from your city, and/or you're willing to connect through any city necessary to get to Dublin), you'll find something. If you aren't (say, you need a nonstop from a city that only has one nonstop a day), then you may be stuck.
I find in my personal checking of fares (from New York) and comparing them with the fares my sister finds (from DC) that fares can start out comparable for our two origin cities, but hers go up sooner than mine do. For instance, with about 6 weeks notice last year, I was able to get a fare of about $650 from New York to Edinburgh, while she got about $950 for the same dates. Just one data point. If you have some flexibility of origin cities, you should check them all. Similarly, could you fly to Shannon instead? Depending on supply and demand for your exact cities and dates, it may be cheaper, or may be available if Dublin is not.