The surname Boylan is of Irish origin and is closely associated with the surnames Boyland and possibly Bolan, Boland, and Boyle. Over time it may also have become associated withother similar surname variants and this may be revealed by DNA testing.

MacLysaght (1) describes the Boylan origins as follows:

O'Boylan, Boyland. The O'Boylan sept of Oriel, which sprang originally from the same stock as the O'Flanagans of Fermanagh, were in early mediaeval times located in a widespread territory stretching from Fermanagh to Louth. Later they were reduced by the MacMahons, but still retained the greater part of the Barony of Dartry in County Monaghan. O'Dugan in his 14th century "topographical poem" praises them for their horsemanship and comments on their blue eyes, calling them "the bold kings of Dartry". They were still more numerous in the Monaghan - Cavan - Meath area than elsewhere. In Irish the nameis Ó Baoighealláin which is etymologically akin to Ó Baoighill, anglice Boyle (q.v.). The prefix O is seldom if ever used with Boylan in modern times, but the alternative form Boyland is sometimes found.

Many Irish family names were anglicized during the Tudor and Elizabethan years of the 16th century according to the motto "if you can't beat 'em, join 'em". Having a name which was easily pronounced by the British colonising powers made life and trade much easier for the local Irish people.  Over time the spelling was shortened to O'Baoilean and the English y was introduced. (The letter y is not known in the Irish language.)  In the 1800s the spelling Boyland was popular and still is used today. Other known spelling variations currently associated with the Boylan Clan are Ó'Baolláin, O'Baoilean, OBoleyn, Boleyn,Boylen, Baylon, Boyland and possibly Bolan. The modern spelling is Boylan.  Considering the Boylan Clan history, DNA testing may reveal close ties to the names above and perhaps also to the Flanagan surname variants, and to other surnames in County Monaghan and surrounding areas. The relationship to Clan Colla is the subject of a specific DNA Project. 

According to MacLysaght (Irish Families, p44-45) the Boyle (Ó' Baoighill) and Boland (Ó'Beolláin) Clans are in fact separate septs / clans and unrelated to the Boylans. One of the purposes of this project is to confirm or refute this with the help of DNA testing. The large and active Boyle Clan has its own Boyle DNA Project. 

The distribution of the Boylan surname (based on Griffith's Valuation) confirms the Monaghan - Cavan-Meath spread of the surname. (2) See the screenshot in the Photos section.

There were 2052 Boylan's in the 1901 Census of Ireland, and 2104 in the 1911 Census. In comparison, the number of Boyland's was 130 and 111 respectively. (3)

Today there are Boylan's all over the world but Ireland still retains the highest concentration of Boylan's with 655 counts per million, followed by the UK with 45, Australia with 40, Canada with 34 and the US with 29. (4)  Further details are found here ...  http://www.boylan.de/Census_2010/census_2010.html

Sources

(1) Irish Families: their Names, Arms and Origins. Edward MacLysaght, 4th edition, 1985, p44

(2) www.irishtimes.com/ancestor/surname

(3) www.census.nationalarchives.ie/search

(4) http://worldnames.publicprofiler.org