Old letters are great fun. If the script seems daunting, that's normal. I find it easiest to start with the first sentence, write down the words that I can read ,with gaps for others, then fill in the gaps. Looking at a word for a bit will often make it legible: it can sort of suddenly appear out of the mist. Capital letters are often the hardest- so just write the other letters, and the other words in the sentence...you can only have "_y dearest _ister", and still pretty reliably have "My dearest Sister ". But really you have something that's in good shape. Most of us have contended with very bad copies, damaged paper, very faded writing...this is actually very good.

Once you've read and maybe even transcribed the letter, so you know what's in it, where it's written from, where was it delivered, you will have a much easier time tracking down more information than simply working with a name. Many of these have the return address at the very top of the page, and it appears this one has been folded so the delivery address is showing. At the very least, if you know where it's from and where it went to, for example, you have two places in which you can search for Ira Allen. Those two places ( town, or county) each likely has a historical society, and that society has likely been asked about most of the families there in the 18th c. already and might be helpful.

Courage!