You can order regimental and overseas certificates from the GRO. Usually this can be done online, but there is clearly a problem with this entry. ‘Ion Is’ are the Ionian Islands, ie Corfu area. They were under British control from c 1816 to the 1860s. I don't know of an Ionian Regiment. There is one reference to the Ionian Volunteers (c1815) on the National Archives web site. But as there is no year you may not be able to order online. You may be able to just enter the year as a range, and you have the Volume and Page references. These indexes are full of errors. I once ordered a birth certificate for the son of a man in the 17th Lancers and it turned out to be the Leicestershire Regiment (aka 17th Foot).
If you have problems you could try phoning them, or write to them with a bigger copy of the entry.
Ken